485BPOS

As filed with the Securities and Exchange Commission on June 5, 2018

1933 Act Registration No. 333-210186

1940 Act Registration No. 811-23147

United States

Securities and Exchange Commission

Washington, D.C. 20549

Form N-1A
 

Registration Statement Under the Securities Act of 1933	[   ]
Pre-Effective Amendment No. __	[   ]
Post-Effective Amendment No. 14	[X]
and/or
Registration Statement Under the Investment Company Act of 1940	[   ]
Amendment No. 16	[X]

First Trust Exchange-Traded Fund VIII

(Exact name of registrant as specified in charter)

120 East Liberty Drive, Suite 400

Wheaton, Illinois 60187

(Address of Principal Executive Offices) (Zip Code)

Registrant’s Telephone Number, including Area Code: (800) 621-1675

W. Scott Jardine, Esq., Secretary

First Trust Exchange-Traded Fund VIII

First Trust Advisors L.P.

120 East Liberty Drive, Suite 400

Wheaton, Illinois 60187

(Name and Address of Agent for Service)

Copy to:

Eric F. Fess, Esq.

Chapman and Cutler LLP

111 West Monroe Street

Chicago, Illinois 60603

It is proposed that this filing will become effective (check appropriate box):

[X] immediately upon filing pursuant to paragraph (b)

[   ] on ___________ pursuant to paragraph (b)

[   ] 60 days after filing pursuant to paragraph (a)(1)

[   ] on (date) pursuant to paragraph (a)(1)

[   ] 75 days after filing pursuant to paragraph (a)(2)

[   ] on (date) pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

[   ] this post-effective amendment designates a new effective date for a previously filed post-effective amendment.

This Registration Statement relates to the First Trust TCW Unconstrained Plus Bond ETF, a series of the Registrant.

Signatures

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement under Rule 485(b) under the Securities Act of 1933 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, duly authorized in the City of Wheaton, and State of Illinois, on the 5th day of June, 2018.

First Trust Exchange-Traded Fund VIII
By:	/s/ James M. Dykas
	James M. Dykas, President and Chief Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the date indicated:

Signature	Title		Date
/s/ James M. Dykas	President and Chief Executive Officer		June 5, 2018
James M. Dykas
/s/ Donald P. Swade	Treasurer, Chief Financial Officer and Chief Accounting Officer		June 5, 2018
Donald P. Swade
James A. Bowen*	) Trustee )
	)
Richard E. Erickson*	) Trustee )
	)
Thomas R. Kadlec*	) Trustee )
	)	By:	/s/ W. Scott Jardine
Robert F. Keith*	) Trustee )		W. Scott Jardine Attorney-In-Fact
	)		June 5, 2018
Niel B. Nielson *	) Trustee )
	)

* Original powers of attorney authorizing W. Scott Jardine, James M. Dykas, Eric F. Fess and Kristi A. Maher to execute Registrant's Registration Statement, and Amendments thereto, for each of the trustees of the Registrant on whose behalf this Registration Statement is filed, were previously executed, and are filed herewith.

Index to Exhibits

(101)       Risk/return summary in interactive data format

XML

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Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Period End Date	dei_DocumentPeriodEndDate	May 29, 2018
Registrant Name	dei_EntityRegistrantName	FIRST TRUST EXCHANGE-TRADED FUND VIII
CIK	dei_EntityCentralIndexKey	0001667919
Amendment Flag	dei_AmendmentFlag	false
Document Created Date	dei_DocumentCreationDate	May 29, 2018
Effective Date	dei_DocumentEffectiveDate	May 29, 2018
Prospectus Date	rr_ProspectusDate	May 29, 2018

XML

First Trust TCW Unconstrained Plus Bond ETF

Summary Information

Investment Objective

The investment objective of the First Trust TCW Unconstrained Plus Bond ETF (the "Fund") is to seek to maximize long-term total return.

Fees and Expenses of the Fund

The following table describes the fees and expenses you may pay if you buy and hold shares of the Fund. Investors purchasing and selling shares may be subject to costs (including customary brokerage commissions) charged by their broker, which are not reflected in the table below.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	First Trust TCW Unconstrained Plus Bond ETF First Trust TCW Unconstrained Plus Bond ETF
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	none

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	First Trust TCW Unconstrained Plus Bond ETF First Trust TCW Unconstrained Plus Bond ETF
Management Fees	0.85%
Distribution and Service (12b-1) Fees	none
Other Expenses	none	[1]
Total Annual Fund Operating Expenses	0.85%
Fee Waiver	0.10%	[2]
Total Annual Fund Operating Expenses After Fee Waiver	0.75%

Example

The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds. This example does not take into account customary brokerage commissions that you pay when purchasing or selling shares of the Fund in the secondary market.

The example assumes that you invest $10,000 in the Fund for the time periods indicated. The example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain at current levels . The example assumes that the Fund's investment advisor's agreement to waive management fees of 0.10% of average daily net assets per year will be terminated following June 5, 2019. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example, No Redemption	1 Year	3 Years
First Trust TCW Unconstrained Plus Bond ETF | First Trust TCW Unconstrained Plus Bond ETF | USD ($)	77	251

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costsand may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund's performance.

Principal Investment Strategies

Under normal market conditions, the Fund intends to invest at least 80% of its net assets (including investment borrowings) in a portfolio of fixed income securities. The Fund's investment sub-advisor, TCW Investment Management Company LLC ("TCW" or the "Sub-Advisor"), will manage the Fund's portfolio in an "unconstrained" manner, meaning that its investment universe is not limited to the securities of any particular index and it has discretion to invest in fixed income securities of any type or credit quality, including up to 70% in high yield (or "junk") securities, up to 60% in emerging market securities and up to 50% in securities denominated in foreign currencies. Additionally, under normal market conditions, the Fund's average portfolio duration will vary from between 0 to 10 years. Duration is a measure of the expected price volatility of a fixed income security as a result of changes in market rates of interest.

In managing the Fund's portfolio, TCW intends to employ a flexible approach that allocates the Fund's investments across a range of global investment opportunities and actively manage exposure to interest rates, credit sectors and currencies. At the core of TCW's investment process is its long-term economic outlook, which drives duration and yield curve and sector positioning. TCW will also seek to utilize independent, bottom-up research to identify securities that are undervalued and that offer a superior risk/return profile.

Pursuant to this investment strategy, the Fund may invest principally in the following types of fixed income securities: securities issued or guaranteed by the U.S. government or its agencies, instrumentalities or U.S. government-sponsored entities; Treasury Inflation Protected Securities ("TIPS"); agency and non-agency residential mortgage-backed securities ("RMBS"); agency and non-agency commercial mortgage-backed securities ("CMBS"); agency and non-agency asset-backed securities ("ABS"); domestic corporate bonds; fixed income securities issued by non-U.S. corporations and non-U.S. governments, including those operating in emerging markets; bank loans, including first lien senior secured floating rate bank loans ("Senior Loans"); and municipal bonds; and other debt securities bearing fixed interest rates of any maturity. The Fund may also invest in collateralized loan obligations ("CLOs"), floating and variable rate securities, Rule 144A securities, preferred stock and common stock.

The Fund may invest a significant portion of its assets in securitized investment products, including up to 50% in each of ABS, RMBS and CMBS. The Fund's investments in mortgage-backed securities may be, but are not required to be, issued or guaranteed by the U.S. government, its agencies or instrumentalities, such as Ginnie Mae and U.S. government-sponsored entities, such as Fannie Mae and Freddie Mac. Government agency or instrumentality securities have different levels of credit support. The Fund may invest in such government supported mortgage-backed securities by investing in to-be-announced transactions ("TBA Transactions"). The Fund may also invest in privately-issued mortgage-backed securities; however, the Fund will not invest more than 20% of its assets invested in fixed income securities in non-agency sponsored mortgage-backed securities.

As stated above, the Fund may invest up to 70% of its net assets in securities rated below investment grade by one or more nationally recognized statistical rating organization ("NRSROs"), or, if unrated, judged to be of comparable quality by the Sub-Advisor (commonly referred to as "high yield" or "junk" bonds). The Fund may also invest up to 60% of its net assets in securities of issuers with significant ties to emerging markets, as determined by the Sub-Advisor. The Sub-Advisor considers emerging market countries to be countries that are characterized by developing commercial and financial infrastructure with significant potential for economic growth and increased capital market participation by non-U.S. investors.

The Fund may also utilize listed and over-the-counter ("OTC") traded derivatives instruments including, but not limited to, futures, options, swap agreements and forward contracts for duration/yield curve management and/or hedging purposes. The Fund will limit the gross notional exposure of its OTC derivatives positions to 20% of its net assets.

The Fund is classified as "non-diversified" under the Investment Company Act of 1940, as amended (the "1940 Act").

Principal Risks

You could lose money by investing in the Fund. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. There can be no assurance that the Fund's investment objective will be achieved.

ASSET-BACKED SECURITIES RISK. Asset-backed securities are bonds or notes backed by loan paper or accounts receivable originated by banks, credit card companies or other providers of credit. Certain asset-backed securities do not have the benefit of the same security interest in the related collateral as do mortgage-backed securities, nor are they provided government guarantees of repayment. Credit card receivables are generally unsecured, and the debtors are entitled to the protection of a number of state and federal consumer credit laws, many of which give such debtors the right to set off certain amounts owed on the credit cards, thereby reducing the balance due. In addition, some issuers of automobile receivables permit the servicers to retain possession of the underlying obligations. If the servicer were to sell these obligations to another party, there is a risk that the purchaser would acquire an interest superior to that of the holders of the related automobile receivables. The impairment of the value of collateral or other assets underlying an asset-backed security, such as a result of non-payment of loans or non-performance of other collateral or underlying assets, may result in a reduction in the value of such asset-backed securities and losses to the Fund.

AUTHORIZED PARTICIPANT CONCENTRATION RISK. Only an authorized participant (as defined in the "Frequent Purchases and Redemptions" Section) may engage in creation or redemption transactions directly with the Fund. The Fund has a limited number of institutions that act as authorized participants. To the extent that these institutions exit the business or are unable to proceed with creation and/or redemption orders with respect to the Fund and no other authorized participant is able to step forward to create or redeem, in either of these cases, Fund shares may trade at a discount to the Fund's net asset value and possibly face delisting.

BANK LOANS RISK. An investment in bank loans subjects the Fund to credit risk, which is heightened for bank loans in which the Fund invests because companies that issue such loans tend to be highly leveraged and thus are more susceptible to the risks of interest deferral, default and/or bankruptcy. Bank loans, in which the Fund may invest, are usually rated below investment grade but may also be unrated. As a result, the risks associated with these bank loans are similar to the risks of high yield fixed income instruments. An economic downturn would generally lead to a higher non-payment rate, and a bank loan may lose significant market value before a default occurs. Moreover, any specific collateral used to secure a bank loan may decline in value or become illiquid, which would adversely affect the bank loan's value. Unlike the securities markets, there is no central clearinghouse for loan trades, and the loan market has not established enforceable settlement standards or remedies for failure to settle. Transactions in loans may settle on a delayed basis, and the Fund may not receive the proceeds from the sale of a loan for a substantial period of time after the sale, which may be longer than seven days in the case of certain Senior Loans. Bank loans are subject to a number of risks described elsewhere in this prospectus, including liquidity risk and the risk of investing in below investment grade fixed income instruments. Furthermore, increases in interest rates may result in greater volatility of bank loans and average duration may fluctuate with fluctuations in interest rates. Unlike stocks and bonds, loans are not registered and otherwise may not be treated as securities under the federal securities laws, meaning investors in loans have less protection against improper practices than investors in securities that are registered under or are otherwise subject to the protections of the securities laws.

CALL RISK. If an issuer calls higher-yielding debt instruments held by the Fund, performance could be adversely impacted.

CASH TRANSACTIONS RISK. The Fund will, under most circumstances, effect a significant portion of creations and redemptions for cash, rather than in-kind securities. As a result, an investment in the Fund may be less tax-efficient than an investment in an exchange-traded fund that effects its creations and redemptions for in-kind securities. Because the Fund may effect a portion of redemptions for cash, it may be required to sell portfolio securities in order to obtain the cash needed to distribute redemption proceeds. A sale of shares may result in capital gains or losses and may also result in higher brokerage costs.

CLO RISK. Certain CLOs may be thinly traded or have a limited trading market. CLOs are typically privately offered and sold. As a result, investments in CLOs may be characterized by the Fund as illiquid securities. In addition to the general risks associated with debt securities discussed herein, CLOs carry additional risks, including, but not limited to: (i) the possibility that distributions from collateral securities will not be adequate to make interest or other payments; (ii) the quality of the collateral may decline in value or default; (iii) the possibility that the investments in CLOs are subordinate to other classes or tranches thereof; and (iv) the complex structure of the security may not be fully understood at the time of investment and may produce disputes with the issuer or unexpected investment results.

COUNTERPARTY RISK. The Fund bears the risk that the counterparty to a contract with a third-party may default on its obligations or otherwise fail to honor its obligations. If a counterparty defaults on its payment obligations, the Fund will lose money and the value of an investment in Fund shares may decrease. In addition, the Fund may engage in such investment transactions with a limited number of counterparties.

CREDIT RISK. Credit risk is the risk that an issuer of a security will be unable or unwilling to make dividend, interest and/or principal payments when due and the related risk that the value of a security may decline because of concerns about the issuer's ability to make such payments. High yield and comparable unrated debt securities, while generally offering higher yields than investment grade debt with similar maturities, involve greater risks, including the possibility of dividend or interest deferral, default or bankruptcy, and are regarded as predominantly speculative with respect to the issuer's capacity to pay dividends or interest and repay principal.

CURRENCY EXCHANGE RATE RISK. The Fund may hold investments that are denominated in non-U.S. currencies, or in securities that provide exposure to such currencies, currency exchange rates or interest rates denominated in such currencies. Changes in currency exchange rates and the relative value of non-U.S. currencies will affect the value of the Fund's investment and the value of Fund shares. Currency exchange rates can be very volatile and can change quickly and unpredictably. As a result, the value of an investment in the Fund may change quickly and without warning and you may lose money.

CURRENCY RISK. Because the Fund's net asset value is determined on the basis of U.S. dollars and the Fund invests in non-U.S. dollar-denominated securities, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the local currency value of the Fund's holdings goes up.

CYBER SECURITY RISK. As the use of Internet technology has become more prevalent in the course of business, the Fund has become more susceptible to potential operational risks through breaches in cyber security. A breach in cyber security refers to both intentional and unintentional events that may cause the Fund to lose proprietary information, suffer data corruption or lose operational capacity. Such events could cause the Fund to incur regulatory penalties, reputational damage, additional compliance costs associated with corrective measures and/or financial loss. Cyber security breaches may involve unauthorized access to the Fund's digital information systems through "hacking" or malicious software coding, but may also result from outside attacks such as denial-of-service attacks through efforts to make network services unavailable to intended users. In addition, cyber security breaches of the Fund's third party service providers, such as its administrator, transfer agent, custodian,  or sub advisor, as applicable, or issuers in which the Fund invests, can also subject the Fund to many of the same risks associated with direct cyber security breaches. The Fund has established risk management systems designed to reduce the risks associated with cyber security. However, there is no guarantee that such efforts will succeed, especially because the Fund does not directly control the cyber security systems of issuers or third party service providers.

DEBT SECURITIES RISK. An investment in the Fund involves risk associated with an investment in debt securities including the risk that certain of the securities in the Fund may not have the benefit of covenants that would prevent the issuer from engaging in capital restructurings or borrowing transactions in connection with corporate acquisitions, leveraged buyouts or restructurings. This limitation could reduce the ability of the issuer to meet its payment obligations and might result in increased credit risk. In addition, certain of the securities may be redeemed or prepaid by the issuer, resulting in lower interest payments received by the Fund and reduced distributions to shareholders.

DERIVATIVES RISK. The Fund may invest in listed and OTC derivatives, including futures, options, swap agreements and forward contracts. The use of derivatives can lead to losses because of adverse movements in the price or value of the underlying asset, index or rate, which may be magnified by certain features of the derivatives. These risks are heightened when the Fund's portfolio managers use derivatives to enhance the Fund's return or as a substitute for a position or security, rather than solely to hedge (or offset) the risk of a position or security held by the Fund.

EMERGING MARKETS RISK. Investments in securities and instruments traded in developing or emerging markets or that provide exposure to such securities or markets, can involve additional risks relating to political, economic, or regulatory conditions not associated with investments in U.S. securities and instruments or investments in more developed international markets. Emerging market governments may, without prior warning, impose capital controls on the ability to transfer currency, securities or other assets. The Fund's ability to access certain developing or emerging markets also may be limited due to a variety of factors, including currency convertibility issues. Such conditions may impact the ability of the Fund to buy, sell or otherwise transfer securities, cause the Fund's returns to differ from those available to domestic investors, adversely affect the trading market and price for Fund shares, and cause the Fund to decline in value.

EXTENSION RISK. Extension risk is the risk that, when interest rates rise, certain obligations will be paid off by the issuer (or obligor) more slowly than anticipated, causing the value of these securities to fall. Rising interest rates tend to extend the duration of securities, making them more sensitive to changes in interest rates. The value of longer-term securities generally changes more in response to changes in interest rates than shorter-term securities. As a result, in a period of rising interest rates, securities may exhibit additional volatility and may lose value.

FLUCTUATION OF NET ASSET VALUE RISK. The net asset value of shares of the Fund will generally fluctuate with changes in the market value of the Fund's holdings. The market prices of shares will generally fluctuate in accordance with changes in net asset value as well as the relative supply of and demand for shares on the Exchange. The Fund's investment advisor cannot predict whether shares will trade below, at or above their net asset value because the shares trade on the Exchange at market prices and not at net asset value. Price differences may be due, in large part, to the fact that supply and demand forces at work in the secondary trading market for shares will be closely related to, but not identical to, the same forces influencing the prices of the holdings of the Fund trading individually or in the aggregate at any point in time. However, given that shares can only be purchased and redeemed either in-kind or for cash in Creation Units, and only to and from broker-dealers and large institutional investors that have entered into participation agreements (unlike shares of closed-end funds, which frequently trade at appreciable discounts from, and sometimes at premiums to, their net asset value), the Fund's investment advisor believes that large discounts or premiums to the net asset value of shares should not be sustained.

HIGH YIELD SECURITIES RISK. High yield securities, or "junk" bonds, are subject to greater market fluctuations and risk of loss than securities with higher ratings, and therefore, are considered to be highly speculative. These securities are issued by issuers that may have narrowly focused operations and/or other impediments to the timely payment of periodic interest and principal at maturity. If the economy slows down or dips into recession, the issuers of high yield securities may not have sufficient resources to continue making timely payment of periodic interest and principal at maturity. The market for high yield securities is smaller and less liquid than that for investment grade securities. High yield securities are generally not listed on a national securities exchange but trade in the over-the-counter markets. Due to the smaller, less liquid market for high yield securities, the bid-offer spread on such securities is generally greater than it is for investment grade securities and the purchase or sale of such securities may take longer to complete. In general, high yield securities may have a greater risk of default than other types of securities.

ILLIQUID SECURITIES RISK. There may be no willing buyer of the securities held by the Fund and the Fund may have to sell those securities at a lower price or may not be able to sell the securities at all, each of which would have a negative impact on Fund performance. The Fund may invest in lower-quality debt instruments issued by issuers that are highly leveraged. Lower-quality debt tends to be less liquid than higher-quality debt. If the economy experiences a sudden downturn, or if the debt markets for such issuers become distressed, the Fund may have particular difficulty selling its assets in sufficient amounts, at reasonable prices and in a sufficiently timely manner to raise the cash necessary to meet any potentially heavy redemption requests by Fund shareholders. In such event, there would be a greater chance that the Fund may be forced to curtail or suspend redemptions, in which case you might experience a delay or inability to liquidate your investment at the desired time or in the desired amount.

INCOME RISK. Income from the Fund's fixed income investments could decline during periods of falling interest rates.

INDEX CONSTITUENT RISK. The Fund may be a constituent of one or more indices. As a result, the Fund may be included in one or more index-tracking exchange-traded funds or mutual funds. Being a component security of such a vehicle could greatly affect the trading activity involving the Fund, the size of the Fund and the market volatility of the Fund. Inclusion in an index could significantly increase demand for the Fund and removal from an index could result in outsized selling activity in a relatively short period of time. As a result, the Fund's net asset value could be negatively impacted and the Fund's market price may be significantly below the Fund's net asset value during certain periods. In addition, index rebalances may potentially result in increased trading activity. To the extent buying or selling activity increases, the Fund can be exposed to increased brokerage costs and adverse tax consequences and the market price of the Fund can be negatively affected.

INFLATION RISK. Inflation risk is the risk that the value of assets or income from investments will be less in the future as inflation decreases the value of money. As inflation increases, the value of the Fund's assets can decline as can the value of the Fund's distributions.

INTEREST RATE RISK. Interest rate risk is the risk that the value of the fixed income securities in the Fund will decline because of rising market interest rates. Interest rate risk is generally lower for shorter term investments and higher for longer term investments. Duration is a measure of the expected price volatility of a debt security as a result of changes in market rates of interest, based on, among other factors, the weighted average timing of the debt security's expected principal and interest payments. In general, duration represents the expected percentage change in the value of a security for an immediate 1% change in interest rates. For example, the price of a security with a three-year duration would be expected to drop by approximately 3% in response to a 1% increase in interest rates. Therefore, prices of debt securities with shorter durations tend to be less sensitive to interest rate changes than debt securities with longer durations. As the value of a debt security changes over time, so will its duration.

MANAGEMENT RISK. The Fund is subject to management risk because it is an actively managed portfolio. In managing the Fund's investment portfolio, the Sub-Advisor will apply investment techniques and risk analyses that may not have the desired result. There can be no guarantee that the Fund will meet its investment objective. In addition, the securities held by the Fund may underperform those held by other funds investing in the same asset class or benchmarks that are representative of the asset class because of the portfolio managers' choice of securities.

MARKET MAKER RISK. If the Fund has lower average daily trading volumes, it may rely on a small number of third-party market makers to provide a market for the purchase and sale of shares. Any trading halt or other problem relating to the trading activity of these market makers could result in a dramatic change in the spread between the Fund's net asset value and the price at which the Fund's shares are trading on the Exchange, which could result in a decrease in value of the Fund's shares. In addition, decisions by market makers or authorized participants to reduce their role or step away from these activities in times of market stress could inhibit the effectiveness of the arbitrage process in maintaining the relationship between the underlying values of the Fund's portfolio securities and the Fund's market price. This reduced effectiveness could result in Fund shares trading at a discount to net asset value and also in greater than normal intraday bid-ask spreads for Fund shares.

MARKET RISK. Market risk is the risk that a particular security owned by the Fund or shares of the Fund in general may fall in value. Securities are subject to market fluctuations caused by such factors as economic, political, regulatory or market developments, changes in interest rates and perceived trends in bond prices. Also, certain of these factors may affect the liquidity of the securities in the Fund's portfolio and make it more difficult for the Fund to sell them. Shares of the Fund could decline in value or underperform other investments.

MORTGAGE SECURITIES RISK. The Fund invests in mortgage-related securities, including mortgage-backed securities, which may make the Fund more susceptible to adverse economic, political or regulatory events that affect the value of real estate. Changes in local, state and federal policies could negatively impact the mortgage-related securities market, which include various government initiated and sponsored homeowner assistance programs and eminent domain issues. Mortgage-related securities may also face liquidity issues when the Fund seeks to sell such securities, but is unable to find buyers at a bid-ask spread to make the transaction feasible. These securities are also subject to the risk that the underlying borrowers may default on their mortgages, resulting in a non-payment of principal and interest. Finally, the mortgage-related securities market may be negatively impacted by regulatory changes including those that are related to the mandate or existence of the government-sponsored enterprises, Fannie Mae, Freddie Mac and Ginnie Mae. Mortgage-related securities are subject to the risk that the rate of mortgage prepayments decreases, which extends the average life of a security and increases the interest rate exposure.

MUNICIPAL OBLIGATIONS RISK. The Fund's investment in municipal securities subjects them to municipal obligations risk. Issuers, including governmental issuers, may be unable to pay their obligations as they come due. The values of municipal obligations that depend on a specific revenue source to fund their payment obligations may fluctuate as a result of actual or anticipated changes in the cash flows generated by the revenue source or changes in the priority of the municipal obligation to receive the cash flows generated by the revenue source. In addition, changes in federal tax laws or the activity of an issuer may adversely affect the tax-exempt status of municipal obligations. Loss of tax-exempt status may cause interest received and distributed to shareholders by the Fund to be taxable and may result in a significant decline in the values of such municipal obligations.

Inventories of municipal securities held by brokers and dealers have decreased in recent years, lessening their ability to make a market in these securities. This reduction in market making capacity has the potential to decrease the Fund's ability to buy or sell municipal securities, and increase price volatility and trading costs, particularly during periods of economic or market stress. In addition, the market for high yield debt has experienced decreased liquidity, and investor perception of increased risk has caused yield spreads to widen. As a result, the Fund may be forced to accept a lower price to sell a municipal security, to sell other securities to raise cash, or to give up an investment opportunity, any of which could have a negative effect on performance.

NON-AGENCY DEBT RISK. Non-agency debt, including RMBS and CMBS that are not issued by a government-sponsored entity such as Fannie Mae, Freddie Mac and Ginnie Mae, are not afforded the protections of backing by the U.S. government. This makes such securities more susceptible to credit, liquidity and other risks discussed in this prospectus.

NON-DIVERSIFICATION RISK. The Fund is classified as "non-diversified" under the 1940 Act. As a result, the Fund is only limited as to the percentage of its assets which may be invested in the securities of any one issuer by the diversification requirements imposed by the Internal Revenue Code of 1986, as amended. The Fund may invest a relatively high percentage of its assets in a limited number of issuers. As a result, the Fund may be more susceptible to a single adverse economic or regulatory occurrence affecting one or more of these issuers, experience increased volatility and be highly invested in certain issuers.

NON-U.S. SECURITIES RISK. Non-U.S. securities are subject to higher volatility than securities of domestic issuers due to possible adverse political, social or economic developments; restrictions on foreign investment or exchange of securities; lack of liquidity; currency exchange rates; excessive taxation; government seizure of assets; different legal or accounting standards; and less government supervision and regulation of exchanges in foreign countries. These risks may be heightened for securities of companies located in, or with significant operations in, emerging market countries.

PORTFOLIO TURNOVER RISK. High portfolio turnover may result in the Fund paying higher levels of transaction costs and generating greater tax liabilities for shareholders. Portfolio turnover risk may cause the Fund's performance to be less than you expect.

PREPAYMENT RISK. The loans and mortgage-related investments in which the Fund invests are subject to pre-payment risk. The degree to which borrowers prepay loans, whether as a contractual requirement or at their election, may be affected by general business conditions, the financial condition of the borrower and competitive conditions among loan investors, among others. As such, prepayments cannot be predicted with accuracy. Upon a prepayment, either in part or in full, the actual outstanding debt on which the Fund derives interest income will be reduced. The Fund may not be able to reinvest the proceeds received on terms as favorable as the prepaid loan.

REPURCHASE AGREEMENT RISK. The Fund's investment in repurchase agreements may be subject to market and credit risk with respect to the collateral securing the repurchase agreements. Investments in repurchase agreements also may be subject to the risk that the market value of the underlying obligations may decline prior to the expiration of the repurchase agreement term.

RESTRICTED SECURITIES RISK. Investments in restricted securities could have the effect of increasing the amount of the Fund's assets invested in illiquid securities if qualified institutional buyers are unwilling to purchase these securities. Illiquid and restricted securities may be difficult to dispose of at the price at which the Fund has valued the securities and at the times when the Fund believes it is desirable to do so. The market price of illiquid and restricted securities generally is more volatile than that of more liquid securities, which may adversely affect the price that the Fund recovers upon the sale of such securities. Investment of the Fund's assets in illiquid and restricted securities may restrict the Fund's ability to take advantage of market opportunities.

SOVEREIGN DEBT RISK. Investments in sovereign bonds involve special risks because the governmental authority that controls the repayment of the debt may be unwilling or unable to repay the principal and/or interest when due in accordance with the terms of such securities; the availability of sufficient foreign exchange on the date a payment is due; the relative size of the debt service burden to the economy as a whole; or the government debtor's policy towards the International Monetary Fund and the political constraints to which a government debtor may be subject. If an issuer of sovereign bonds defaults on payments of principal and/or interest, the Fund may have limited legal recourse against the issuer and/or guarantor. In certain cases, remedies must be pursued in the courts of the defaulting party itself, and the Fund's ability to obtain recourse may be limited. In times of economic uncertainty, the prices of these securities may be more volatile than those of corporate debt obligations or of other government debt obligations.

SWAP AGREEMENTS RISK. In addition to risks applicable to derivatives generally, the use of swap agreements subjects the Fund to following additional risks: (1) the inability to assign a swap contract without the consent of the counterparty; (2) potential default of the counterparty to a swap; (3) absence of a liquid secondary market for any particular swap at any time; and (4) possible inability of the Fund to close out a swap transaction at a time that otherwise would be favorable for it to do so.

TBA TRANSACTIONS RISK. TBA Transactions involve a risk of loss if the value of the underlying security to be purchased by the Fund declines in value prior to the delivery date. Such transactions are also subject to the risk of the default or bankruptcy of the Fund's counterparty or that the security will not be issued. If this occurs, the Fund loses both the investment opportunity for the assets it set aside to pay for the security and any gain in the security's price.

TIPS RISK. TIPs are inflation-indexed fixed income securities issued by the U.S. Department of Treasury and are subject to the same risks as other fixed income investments. In a falling inflationary environment, both interest payments and the value of the TIPS will decline.

TRADING ISSUES RISK. Although the shares of the Fund are listed for trading on the Exchange, there can be no assurance that an active trading market for such shares will develop or be maintained. Trading in shares on the Exchange may be halted due to market conditions or for reasons that, in the view of the Exchange, make trading in shares inadvisable. In addition, trading in shares on the Exchange is subject to trading halts caused by extraordinary market volatility pursuant to the Exchange's "circuit breaker" rules. Market makers are under no obligation to make a market in the Fund's shares, and authorized participants are not obligated to submit purchase or redemption orders for Creation Units. In the event market makers cease making a market in the Fund's shares or authorized participants stop submitting purchase or redemption orders for Creation Units, Fund shares may trade at a larger premium or discount to their net asset value. There can be no assurance that the requirements of the Exchange necessary to maintain the listing of the Fund will continue to be met or will remain unchanged. The Fund may have difficulty maintaining its listing on the Exchange in the event the Fund's assets are small or the Fund does not have enough shareholders.

U.S. GOVERNMENT AND AGENCY SECURITIES RISK. U.S. government obligations include U.S. Treasury obligations and securities issued or guaranteed by various agencies of the U.S. government or by various instrumentalities, which have been established or sponsored by the U.S. government. U.S. Treasury obligations are backed by the "full faith and credit" of the U.S. government. Securities issued or guaranteed by federal agencies and U.S. government sponsored instrumentalities may or may not be backed by the full faith and credit of the U.S. government.

Performance

The Fund has not yet operated for a full calendar year and, therefore, performance information is not included in this section of the prospectus. See "Total Return Information" for performance information regarding the Fund.

[1] "Other Expenses" is an estimate based on the expenses the Fund expects to incur for the current fiscal year.

[2] Pursuant to a contractual agreement, First Trust Advisors L.P., the Fund's investment advisor, has agreed to waive management fees of 0.10% of average daily net assets until June 5, 2019. The waiver agreement may be terminated by action of the Trust's Board of Trustees at any time upon 60 days' written notice by the Trust on behalf of the Fund or by the Fund's investment advisor only after June 5, 2019.

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Registrant Name

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FIRST TRUST EXCHANGE-TRADED FUND VIII

Prospectus Date

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May 29, 2018

First Trust TCW Unconstrained Plus Bond ETF

Risk/Return:

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Summary Information

Objective [Heading]

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Investment Objective

Objective, Primary [Text Block]

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The investment objective of the First Trust TCW Unconstrained Plus Bond ETF (the "Fund") is to seek to maximize long-term total return.

Expense [Heading]

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Fees and Expenses of the Fund

Expense Narrative [Text Block]

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The following table describes the fees and expenses you may pay if you buy and hold shares of the Fund. Investors purchasing and selling shares may be subject to costs (including customary brokerage commissions) charged by their broker, which are not reflected in the table below.

Shareholder Fees Caption [Text]

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Shareholder Fees
(fees paid directly from your investment)

Operating Expenses Caption [Text]

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Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

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Portfolio Turnover

Portfolio Turnover [Text Block]

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The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costsand may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund's performance.

Expense Example [Heading]

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Example

Expense Example, No Redemption Narrative [Text Block]

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The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds. This example does not take into account customary brokerage commissions that you pay when purchasing or selling shares of the Fund in the secondary market.

The example assumes that you invest $10,000 in the Fund for the time periods indicated. The example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain at current levels . The example assumes that the Fund's investment advisor's agreement to waive management fees of 0.10% of average daily net assets per year will be terminated following June 5, 2019. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

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Principal Investment Strategies

Strategy Narrative [Text Block]

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Under normal market conditions, the Fund intends to invest at least 80% of its net assets (including investment borrowings) in a portfolio of fixed income securities. The Fund's investment sub-advisor, TCW Investment Management Company LLC ("TCW" or the "Sub-Advisor"), will manage the Fund's portfolio in an "unconstrained" manner, meaning that its investment universe is not limited to the securities of any particular index and it has discretion to invest in fixed income securities of any type or credit quality, including up to 70% in high yield (or "junk") securities, up to 60% in emerging market securities and up to 50% in securities denominated in foreign currencies. Additionally, under normal market conditions, the Fund's average portfolio duration will vary from between 0 to 10 years. Duration is a measure of the expected price volatility of a fixed income security as a result of changes in market rates of interest.

In managing the Fund's portfolio, TCW intends to employ a flexible approach that allocates the Fund's investments across a range of global investment opportunities and actively manage exposure to interest rates, credit sectors and currencies. At the core of TCW's investment process is its long-term economic outlook, which drives duration and yield curve and sector positioning. TCW will also seek to utilize independent, bottom-up research to identify securities that are undervalued and that offer a superior risk/return profile.

Pursuant to this investment strategy, the Fund may invest principally in the following types of fixed income securities: securities issued or guaranteed by the U.S. government or its agencies, instrumentalities or U.S. government-sponsored entities; Treasury Inflation Protected Securities ("TIPS"); agency and non-agency residential mortgage-backed securities ("RMBS"); agency and non-agency commercial mortgage-backed securities ("CMBS"); agency and non-agency asset-backed securities ("ABS"); domestic corporate bonds; fixed income securities issued by non-U.S. corporations and non-U.S. governments, including those operating in emerging markets; bank loans, including first lien senior secured floating rate bank loans ("Senior Loans"); and municipal bonds; and other debt securities bearing fixed interest rates of any maturity. The Fund may also invest in collateralized loan obligations ("CLOs"), floating and variable rate securities, Rule 144A securities, preferred stock and common stock.

The Fund may invest a significant portion of its assets in securitized investment products, including up to 50% in each of ABS, RMBS and CMBS. The Fund's investments in mortgage-backed securities may be, but are not required to be, issued or guaranteed by the U.S. government, its agencies or instrumentalities, such as Ginnie Mae and U.S. government-sponsored entities, such as Fannie Mae and Freddie Mac. Government agency or instrumentality securities have different levels of credit support. The Fund may invest in such government supported mortgage-backed securities by investing in to-be-announced transactions ("TBA Transactions"). The Fund may also invest in privately-issued mortgage-backed securities; however, the Fund will not invest more than 20% of its assets invested in fixed income securities in non-agency sponsored mortgage-backed securities.

As stated above, the Fund may invest up to 70% of its net assets in securities rated below investment grade by one or more nationally recognized statistical rating organization ("NRSROs"), or, if unrated, judged to be of comparable quality by the Sub-Advisor (commonly referred to as "high yield" or "junk" bonds). The Fund may also invest up to 60% of its net assets in securities of issuers with significant ties to emerging markets, as determined by the Sub-Advisor. The Sub-Advisor considers emerging market countries to be countries that are characterized by developing commercial and financial infrastructure with significant potential for economic growth and increased capital market participation by non-U.S. investors.

The Fund may also utilize listed and over-the-counter ("OTC") traded derivatives instruments including, but not limited to, futures, options, swap agreements and forward contracts for duration/yield curve management and/or hedging purposes. The Fund will limit the gross notional exposure of its OTC derivatives positions to 20% of its net assets.

The Fund is classified as "non-diversified" under the Investment Company Act of 1940, as amended (the "1940 Act").

Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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You could lose money by investing in the Fund. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. There can be no assurance that the Fund's investment objective will be achieved.

ASSET-BACKED SECURITIES RISK. Asset-backed securities are bonds or notes backed by loan paper or accounts receivable originated by banks, credit card companies or other providers of credit. Certain asset-backed securities do not have the benefit of the same security interest in the related collateral as do mortgage-backed securities, nor are they provided government guarantees of repayment. Credit card receivables are generally unsecured, and the debtors are entitled to the protection of a number of state and federal consumer credit laws, many of which give such debtors the right to set off certain amounts owed on the credit cards, thereby reducing the balance due. In addition, some issuers of automobile receivables permit the servicers to retain possession of the underlying obligations. If the servicer were to sell these obligations to another party, there is a risk that the purchaser would acquire an interest superior to that of the holders of the related automobile receivables. The impairment of the value of collateral or other assets underlying an asset-backed security, such as a result of non-payment of loans or non-performance of other collateral or underlying assets, may result in a reduction in the value of such asset-backed securities and losses to the Fund.

AUTHORIZED PARTICIPANT CONCENTRATION RISK. Only an authorized participant (as defined in the "Frequent Purchases and Redemptions" Section) may engage in creation or redemption transactions directly with the Fund. The Fund has a limited number of institutions that act as authorized participants. To the extent that these institutions exit the business or are unable to proceed with creation and/or redemption orders with respect to the Fund and no other authorized participant is able to step forward to create or redeem, in either of these cases, Fund shares may trade at a discount to the Fund's net asset value and possibly face delisting.

BANK LOANS RISK. An investment in bank loans subjects the Fund to credit risk, which is heightened for bank loans in which the Fund invests because companies that issue such loans tend to be highly leveraged and thus are more susceptible to the risks of interest deferral, default and/or bankruptcy. Bank loans, in which the Fund may invest, are usually rated below investment grade but may also be unrated. As a result, the risks associated with these bank loans are similar to the risks of high yield fixed income instruments. An economic downturn would generally lead to a higher non-payment rate, and a bank loan may lose significant market value before a default occurs. Moreover, any specific collateral used to secure a bank loan may decline in value or become illiquid, which would adversely affect the bank loan's value. Unlike the securities markets, there is no central clearinghouse for loan trades, and the loan market has not established enforceable settlement standards or remedies for failure to settle. Transactions in loans may settle on a delayed basis, and the Fund may not receive the proceeds from the sale of a loan for a substantial period of time after the sale, which may be longer than seven days in the case of certain Senior Loans. Bank loans are subject to a number of risks described elsewhere in this prospectus, including liquidity risk and the risk of investing in below investment grade fixed income instruments. Furthermore, increases in interest rates may result in greater volatility of bank loans and average duration may fluctuate with fluctuations in interest rates. Unlike stocks and bonds, loans are not registered and otherwise may not be treated as securities under the federal securities laws, meaning investors in loans have less protection against improper practices than investors in securities that are registered under or are otherwise subject to the protections of the securities laws.

CALL RISK. If an issuer calls higher-yielding debt instruments held by the Fund, performance could be adversely impacted.

CASH TRANSACTIONS RISK. The Fund will, under most circumstances, effect a significant portion of creations and redemptions for cash, rather than in-kind securities. As a result, an investment in the Fund may be less tax-efficient than an investment in an exchange-traded fund that effects its creations and redemptions for in-kind securities. Because the Fund may effect a portion of redemptions for cash, it may be required to sell portfolio securities in order to obtain the cash needed to distribute redemption proceeds. A sale of shares may result in capital gains or losses and may also result in higher brokerage costs.

CLO RISK. Certain CLOs may be thinly traded or have a limited trading market. CLOs are typically privately offered and sold. As a result, investments in CLOs may be characterized by the Fund as illiquid securities. In addition to the general risks associated with debt securities discussed herein, CLOs carry additional risks, including, but not limited to: (i) the possibility that distributions from collateral securities will not be adequate to make interest or other payments; (ii) the quality of the collateral may decline in value or default; (iii) the possibility that the investments in CLOs are subordinate to other classes or tranches thereof; and (iv) the complex structure of the security may not be fully understood at the time of investment and may produce disputes with the issuer or unexpected investment results.

COUNTERPARTY RISK. The Fund bears the risk that the counterparty to a contract with a third-party may default on its obligations or otherwise fail to honor its obligations. If a counterparty defaults on its payment obligations, the Fund will lose money and the value of an investment in Fund shares may decrease. In addition, the Fund may engage in such investment transactions with a limited number of counterparties.

CREDIT RISK. Credit risk is the risk that an issuer of a security will be unable or unwilling to make dividend, interest and/or principal payments when due and the related risk that the value of a security may decline because of concerns about the issuer's ability to make such payments. High yield and comparable unrated debt securities, while generally offering higher yields than investment grade debt with similar maturities, involve greater risks, including the possibility of dividend or interest deferral, default or bankruptcy, and are regarded as predominantly speculative with respect to the issuer's capacity to pay dividends or interest and repay principal.

CURRENCY EXCHANGE RATE RISK. The Fund may hold investments that are denominated in non-U.S. currencies, or in securities that provide exposure to such currencies, currency exchange rates or interest rates denominated in such currencies. Changes in currency exchange rates and the relative value of non-U.S. currencies will affect the value of the Fund's investment and the value of Fund shares. Currency exchange rates can be very volatile and can change quickly and unpredictably. As a result, the value of an investment in the Fund may change quickly and without warning and you may lose money.

CURRENCY RISK. Because the Fund's net asset value is determined on the basis of U.S. dollars and the Fund invests in non-U.S. dollar-denominated securities, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the local currency value of the Fund's holdings goes up.

CYBER SECURITY RISK. As the use of Internet technology has become more prevalent in the course of business, the Fund has become more susceptible to potential operational risks through breaches in cyber security. A breach in cyber security refers to both intentional and unintentional events that may cause the Fund to lose proprietary information, suffer data corruption or lose operational capacity. Such events could cause the Fund to incur regulatory penalties, reputational damage, additional compliance costs associated with corrective measures and/or financial loss. Cyber security breaches may involve unauthorized access to the Fund's digital information systems through "hacking" or malicious software coding, but may also result from outside attacks such as denial-of-service attacks through efforts to make network services unavailable to intended users. In addition, cyber security breaches of the Fund's third party service providers, such as its administrator, transfer agent, custodian,  or sub advisor, as applicable, or issuers in which the Fund invests, can also subject the Fund to many of the same risks associated with direct cyber security breaches. The Fund has established risk management systems designed to reduce the risks associated with cyber security. However, there is no guarantee that such efforts will succeed, especially because the Fund does not directly control the cyber security systems of issuers or third party service providers.

DEBT SECURITIES RISK. An investment in the Fund involves risk associated with an investment in debt securities including the risk that certain of the securities in the Fund may not have the benefit of covenants that would prevent the issuer from engaging in capital restructurings or borrowing transactions in connection with corporate acquisitions, leveraged buyouts or restructurings. This limitation could reduce the ability of the issuer to meet its payment obligations and might result in increased credit risk. In addition, certain of the securities may be redeemed or prepaid by the issuer, resulting in lower interest payments received by the Fund and reduced distributions to shareholders.

DERIVATIVES RISK. The Fund may invest in listed and OTC derivatives, including futures, options, swap agreements and forward contracts. The use of derivatives can lead to losses because of adverse movements in the price or value of the underlying asset, index or rate, which may be magnified by certain features of the derivatives. These risks are heightened when the Fund's portfolio managers use derivatives to enhance the Fund's return or as a substitute for a position or security, rather than solely to hedge (or offset) the risk of a position or security held by the Fund.

EMERGING MARKETS RISK. Investments in securities and instruments traded in developing or emerging markets or that provide exposure to such securities or markets, can involve additional risks relating to political, economic, or regulatory conditions not associated with investments in U.S. securities and instruments or investments in more developed international markets. Emerging market governments may, without prior warning, impose capital controls on the ability to transfer currency, securities or other assets. The Fund's ability to access certain developing or emerging markets also may be limited due to a variety of factors, including currency convertibility issues. Such conditions may impact the ability of the Fund to buy, sell or otherwise transfer securities, cause the Fund's returns to differ from those available to domestic investors, adversely affect the trading market and price for Fund shares, and cause the Fund to decline in value.

EXTENSION RISK. Extension risk is the risk that, when interest rates rise, certain obligations will be paid off by the issuer (or obligor) more slowly than anticipated, causing the value of these securities to fall. Rising interest rates tend to extend the duration of securities, making them more sensitive to changes in interest rates. The value of longer-term securities generally changes more in response to changes in interest rates than shorter-term securities. As a result, in a period of rising interest rates, securities may exhibit additional volatility and may lose value.

FLUCTUATION OF NET ASSET VALUE RISK. The net asset value of shares of the Fund will generally fluctuate with changes in the market value of the Fund's holdings. The market prices of shares will generally fluctuate in accordance with changes in net asset value as well as the relative supply of and demand for shares on the Exchange. The Fund's investment advisor cannot predict whether shares will trade below, at or above their net asset value because the shares trade on the Exchange at market prices and not at net asset value. Price differences may be due, in large part, to the fact that supply and demand forces at work in the secondary trading market for shares will be closely related to, but not identical to, the same forces influencing the prices of the holdings of the Fund trading individually or in the aggregate at any point in time. However, given that shares can only be purchased and redeemed either in-kind or for cash in Creation Units, and only to and from broker-dealers and large institutional investors that have entered into participation agreements (unlike shares of closed-end funds, which frequently trade at appreciable discounts from, and sometimes at premiums to, their net asset value), the Fund's investment advisor believes that large discounts or premiums to the net asset value of shares should not be sustained.

HIGH YIELD SECURITIES RISK. High yield securities, or "junk" bonds, are subject to greater market fluctuations and risk of loss than securities with higher ratings, and therefore, are considered to be highly speculative. These securities are issued by issuers that may have narrowly focused operations and/or other impediments to the timely payment of periodic interest and principal at maturity. If the economy slows down or dips into recession, the issuers of high yield securities may not have sufficient resources to continue making timely payment of periodic interest and principal at maturity. The market for high yield securities is smaller and less liquid than that for investment grade securities. High yield securities are generally not listed on a national securities exchange but trade in the over-the-counter markets. Due to the smaller, less liquid market for high yield securities, the bid-offer spread on such securities is generally greater than it is for investment grade securities and the purchase or sale of such securities may take longer to complete. In general, high yield securities may have a greater risk of default than other types of securities.

ILLIQUID SECURITIES RISK. There may be no willing buyer of the securities held by the Fund and the Fund may have to sell those securities at a lower price or may not be able to sell the securities at all, each of which would have a negative impact on Fund performance. The Fund may invest in lower-quality debt instruments issued by issuers that are highly leveraged. Lower-quality debt tends to be less liquid than higher-quality debt. If the economy experiences a sudden downturn, or if the debt markets for such issuers become distressed, the Fund may have particular difficulty selling its assets in sufficient amounts, at reasonable prices and in a sufficiently timely manner to raise the cash necessary to meet any potentially heavy redemption requests by Fund shareholders. In such event, there would be a greater chance that the Fund may be forced to curtail or suspend redemptions, in which case you might experience a delay or inability to liquidate your investment at the desired time or in the desired amount.

INCOME RISK. Income from the Fund's fixed income investments could decline during periods of falling interest rates.

INDEX CONSTITUENT RISK. The Fund may be a constituent of one or more indices. As a result, the Fund may be included in one or more index-tracking exchange-traded funds or mutual funds. Being a component security of such a vehicle could greatly affect the trading activity involving the Fund, the size of the Fund and the market volatility of the Fund. Inclusion in an index could significantly increase demand for the Fund and removal from an index could result in outsized selling activity in a relatively short period of time. As a result, the Fund's net asset value could be negatively impacted and the Fund's market price may be significantly below the Fund's net asset value during certain periods. In addition, index rebalances may potentially result in increased trading activity. To the extent buying or selling activity increases, the Fund can be exposed to increased brokerage costs and adverse tax consequences and the market price of the Fund can be negatively affected.

INFLATION RISK. Inflation risk is the risk that the value of assets or income from investments will be less in the future as inflation decreases the value of money. As inflation increases, the value of the Fund's assets can decline as can the value of the Fund's distributions.

INTEREST RATE RISK. Interest rate risk is the risk that the value of the fixed income securities in the Fund will decline because of rising market interest rates. Interest rate risk is generally lower for shorter term investments and higher for longer term investments. Duration is a measure of the expected price volatility of a debt security as a result of changes in market rates of interest, based on, among other factors, the weighted average timing of the debt security's expected principal and interest payments. In general, duration represents the expected percentage change in the value of a security for an immediate 1% change in interest rates. For example, the price of a security with a three-year duration would be expected to drop by approximately 3% in response to a 1% increase in interest rates. Therefore, prices of debt securities with shorter durations tend to be less sensitive to interest rate changes than debt securities with longer durations. As the value of a debt security changes over time, so will its duration.

MANAGEMENT RISK. The Fund is subject to management risk because it is an actively managed portfolio. In managing the Fund's investment portfolio, the Sub-Advisor will apply investment techniques and risk analyses that may not have the desired result. There can be no guarantee that the Fund will meet its investment objective. In addition, the securities held by the Fund may underperform those held by other funds investing in the same asset class or benchmarks that are representative of the asset class because of the portfolio managers' choice of securities.

MARKET MAKER RISK. If the Fund has lower average daily trading volumes, it may rely on a small number of third-party market makers to provide a market for the purchase and sale of shares. Any trading halt or other problem relating to the trading activity of these market makers could result in a dramatic change in the spread between the Fund's net asset value and the price at which the Fund's shares are trading on the Exchange, which could result in a decrease in value of the Fund's shares. In addition, decisions by market makers or authorized participants to reduce their role or step away from these activities in times of market stress could inhibit the effectiveness of the arbitrage process in maintaining the relationship between the underlying values of the Fund's portfolio securities and the Fund's market price. This reduced effectiveness could result in Fund shares trading at a discount to net asset value and also in greater than normal intraday bid-ask spreads for Fund shares.

MARKET RISK. Market risk is the risk that a particular security owned by the Fund or shares of the Fund in general may fall in value. Securities are subject to market fluctuations caused by such factors as economic, political, regulatory or market developments, changes in interest rates and perceived trends in bond prices. Also, certain of these factors may affect the liquidity of the securities in the Fund's portfolio and make it more difficult for the Fund to sell them. Shares of the Fund could decline in value or underperform other investments.

MORTGAGE SECURITIES RISK. The Fund invests in mortgage-related securities, including mortgage-backed securities, which may make the Fund more susceptible to adverse economic, political or regulatory events that affect the value of real estate. Changes in local, state and federal policies could negatively impact the mortgage-related securities market, which include various government initiated and sponsored homeowner assistance programs and eminent domain issues. Mortgage-related securities may also face liquidity issues when the Fund seeks to sell such securities, but is unable to find buyers at a bid-ask spread to make the transaction feasible. These securities are also subject to the risk that the underlying borrowers may default on their mortgages, resulting in a non-payment of principal and interest. Finally, the mortgage-related securities market may be negatively impacted by regulatory changes including those that are related to the mandate or existence of the government-sponsored enterprises, Fannie Mae, Freddie Mac and Ginnie Mae. Mortgage-related securities are subject to the risk that the rate of mortgage prepayments decreases, which extends the average life of a security and increases the interest rate exposure.

MUNICIPAL OBLIGATIONS RISK. The Fund's investment in municipal securities subjects them to municipal obligations risk. Issuers, including governmental issuers, may be unable to pay their obligations as they come due. The values of municipal obligations that depend on a specific revenue source to fund their payment obligations may fluctuate as a result of actual or anticipated changes in the cash flows generated by the revenue source or changes in the priority of the municipal obligation to receive the cash flows generated by the revenue source. In addition, changes in federal tax laws or the activity of an issuer may adversely affect the tax-exempt status of municipal obligations. Loss of tax-exempt status may cause interest received and distributed to shareholders by the Fund to be taxable and may result in a significant decline in the values of such municipal obligations.

Inventories of municipal securities held by brokers and dealers have decreased in recent years, lessening their ability to make a market in these securities. This reduction in market making capacity has the potential to decrease the Fund's ability to buy or sell municipal securities, and increase price volatility and trading costs, particularly during periods of economic or market stress. In addition, the market for high yield debt has experienced decreased liquidity, and investor perception of increased risk has caused yield spreads to widen. As a result, the Fund may be forced to accept a lower price to sell a municipal security, to sell other securities to raise cash, or to give up an investment opportunity, any of which could have a negative effect on performance.

NON-AGENCY DEBT RISK. Non-agency debt, including RMBS and CMBS that are not issued by a government-sponsored entity such as Fannie Mae, Freddie Mac and Ginnie Mae, are not afforded the protections of backing by the U.S. government. This makes such securities more susceptible to credit, liquidity and other risks discussed in this prospectus.

NON-DIVERSIFICATION RISK. The Fund is classified as "non-diversified" under the 1940 Act. As a result, the Fund is only limited as to the percentage of its assets which may be invested in the securities of any one issuer by the diversification requirements imposed by the Internal Revenue Code of 1986, as amended. The Fund may invest a relatively high percentage of its assets in a limited number of issuers. As a result, the Fund may be more susceptible to a single adverse economic or regulatory occurrence affecting one or more of these issuers, experience increased volatility and be highly invested in certain issuers.

NON-U.S. SECURITIES RISK. Non-U.S. securities are subject to higher volatility than securities of domestic issuers due to possible adverse political, social or economic developments; restrictions on foreign investment or exchange of securities; lack of liquidity; currency exchange rates; excessive taxation; government seizure of assets; different legal or accounting standards; and less government supervision and regulation of exchanges in foreign countries. These risks may be heightened for securities of companies located in, or with significant operations in, emerging market countries.

PORTFOLIO TURNOVER RISK. High portfolio turnover may result in the Fund paying higher levels of transaction costs and generating greater tax liabilities for shareholders. Portfolio turnover risk may cause the Fund's performance to be less than you expect.

PREPAYMENT RISK. The loans and mortgage-related investments in which the Fund invests are subject to pre-payment risk. The degree to which borrowers prepay loans, whether as a contractual requirement or at their election, may be affected by general business conditions, the financial condition of the borrower and competitive conditions among loan investors, among others. As such, prepayments cannot be predicted with accuracy. Upon a prepayment, either in part or in full, the actual outstanding debt on which the Fund derives interest income will be reduced. The Fund may not be able to reinvest the proceeds received on terms as favorable as the prepaid loan.

REPURCHASE AGREEMENT RISK. The Fund's investment in repurchase agreements may be subject to market and credit risk with respect to the collateral securing the repurchase agreements. Investments in repurchase agreements also may be subject to the risk that the market value of the underlying obligations may decline prior to the expiration of the repurchase agreement term.

RESTRICTED SECURITIES RISK. Investments in restricted securities could have the effect of increasing the amount of the Fund's assets invested in illiquid securities if qualified institutional buyers are unwilling to purchase these securities. Illiquid and restricted securities may be difficult to dispose of at the price at which the Fund has valued the securities and at the times when the Fund believes it is desirable to do so. The market price of illiquid and restricted securities generally is more volatile than that of more liquid securities, which may adversely affect the price that the Fund recovers upon the sale of such securities. Investment of the Fund's assets in illiquid and restricted securities may restrict the Fund's ability to take advantage of market opportunities.

SOVEREIGN DEBT RISK. Investments in sovereign bonds involve special risks because the governmental authority that controls the repayment of the debt may be unwilling or unable to repay the principal and/or interest when due in accordance with the terms of such securities; the availability of sufficient foreign exchange on the date a payment is due; the relative size of the debt service burden to the economy as a whole; or the government debtor's policy towards the International Monetary Fund and the political constraints to which a government debtor may be subject. If an issuer of sovereign bonds defaults on payments of principal and/or interest, the Fund may have limited legal recourse against the issuer and/or guarantor. In certain cases, remedies must be pursued in the courts of the defaulting party itself, and the Fund's ability to obtain recourse may be limited. In times of economic uncertainty, the prices of these securities may be more volatile than those of corporate debt obligations or of other government debt obligations.

SWAP AGREEMENTS RISK. In addition to risks applicable to derivatives generally, the use of swap agreements subjects the Fund to following additional risks: (1) the inability to assign a swap contract without the consent of the counterparty; (2) potential default of the counterparty to a swap; (3) absence of a liquid secondary market for any particular swap at any time; and (4) possible inability of the Fund to close out a swap transaction at a time that otherwise would be favorable for it to do so.

TBA TRANSACTIONS RISK. TBA Transactions involve a risk of loss if the value of the underlying security to be purchased by the Fund declines in value prior to the delivery date. Such transactions are also subject to the risk of the default or bankruptcy of the Fund's counterparty or that the security will not be issued. If this occurs, the Fund loses both the investment opportunity for the assets it set aside to pay for the security and any gain in the security's price.

TIPS RISK. TIPs are inflation-indexed fixed income securities issued by the U.S. Department of Treasury and are subject to the same risks as other fixed income investments. In a falling inflationary environment, both interest payments and the value of the TIPS will decline.

TRADING ISSUES RISK. Although the shares of the Fund are listed for trading on the Exchange, there can be no assurance that an active trading market for such shares will develop or be maintained. Trading in shares on the Exchange may be halted due to market conditions or for reasons that, in the view of the Exchange, make trading in shares inadvisable. In addition, trading in shares on the Exchange is subject to trading halts caused by extraordinary market volatility pursuant to the Exchange's "circuit breaker" rules. Market makers are under no obligation to make a market in the Fund's shares, and authorized participants are not obligated to submit purchase or redemption orders for Creation Units. In the event market makers cease making a market in the Fund's shares or authorized participants stop submitting purchase or redemption orders for Creation Units, Fund shares may trade at a larger premium or discount to their net asset value. There can be no assurance that the requirements of the Exchange necessary to maintain the listing of the Fund will continue to be met or will remain unchanged. The Fund may have difficulty maintaining its listing on the Exchange in the event the Fund's assets are small or the Fund does not have enough shareholders.

U.S. GOVERNMENT AND AGENCY SECURITIES RISK. U.S. government obligations include U.S. Treasury obligations and securities issued or guaranteed by various agencies of the U.S. government or by various instrumentalities, which have been established or sponsored by the U.S. government. U.S. Treasury obligations are backed by the "full faith and credit" of the U.S. government. Securities issued or guaranteed by federal agencies and U.S. government sponsored instrumentalities may or may not be backed by the full faith and credit of the U.S. government.

Bar Chart and Performance Table [Heading]

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Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The Fund has not yet operated for a full calendar year and, therefore, performance information is not included in this section of the prospectus. See "Total Return Information" for performance information regarding the Fund.

First Trust TCW Unconstrained Plus Bond ETF | First Trust TCW Unconstrained Plus Bond ETF

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Management Fees

rr_ManagementFeesOverAssets

0.85%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

none

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.85%

Fee Waiver

rr_FeeWaiverOrReimbursementOverAssets

0.10%

^[2]

Total Annual Fund Operating Expenses After Fee Waiver

rr_NetExpensesOverAssets

0.75%

1 Year

rr_ExpenseExampleNoRedemptionYear01

$ 77

3 Years

rr_ExpenseExampleNoRedemptionYear03

$ 251

[1] "Other Expenses" is an estimate based on the expenses the Fund expects to incur for the current fiscal year.

[2] Pursuant to a contractual agreement, First Trust Advisors L.P., the Fund's investment advisor, has agreed to waive management fees of 0.10% of average daily net assets until June 5, 2019. The waiver agreement may be terminated by action of the Trust's Board of Trustees at any time upon 60 days' written notice by the Trust on behalf of the Fund or by the Fund's investment advisor only after June 5, 2019.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIRST TRUST EXCHANGE-TRADED FUND VIII
Prospectus Date	rr_ProspectusDate	May 29, 2018
Document Creation Date	dei_DocumentCreationDate	May 29, 2018