<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-04-12</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000100591</issuerCik>
        <issuerName>ARGAN INC</issuerName>
        <issuerTradingSymbol>AGX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001482442</rptOwnerCik>
            <rptOwnerName>Watson David Hibbert</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ARGAN, INC.,</rptOwnerStreet1>
            <rptOwnerStreet2>ONE CHURCH STREET, SUITE 201</rptOwnerStreet2>
            <rptOwnerCity>ROCKVILLE</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>20850</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP &amp; CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2700</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option to purchase Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>50.30</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-04-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>32000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2020-04-12</value>
            </exerciseDate>
            <expirationDate>
                <value>2029-04-12</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>32000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>134000</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Performance-Based Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0.00</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-04-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>8000</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>8000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16000</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On April 12, 2019, the Reporting Person received 10-year options to purchase 32,000 shares of the Issuer's common stock with an exercise price of $50.30 per share. The options will vest ratably over three years on each anniversary of the grant date starting from 04/12/2020.</footnote>
        <footnote id="F2">As of the date of this filing, the total number of options owned by the Reporting Person includes (a) options to acquire 30,000 shares of common stock at a price of $41.68 with Date Exercisable of 06/23/2017 and Expiration Date of 06/23/2026; (b) options to acquire 40,000 shares of common stock at a price of $64.25 per share with Date Exercisable of 04/06/2018 and Expiration Date of 04/06/2027; (c) options to acquire 32,000 shares of common stock at a price of $37.60 per share, subject to a three-year ratably vesting schedule, with Date Exercisable of 04/13/2019 and Expiration Date of 04/13/2028; and (d) options to acquire 32,000 shares of common stock at a price of $50.30 subject to the aforementioned three-year ratably vesting schedule with a Date Exercisable of 4/12/2020 and Expiration date of 4/12/2029.</footnote>
        <footnote id="F3">On April 12, 2019, the Reporting Person was granted Performance-Based Restricted Stock Units (the &quot;PBRSUs&quot;) in the target number of 8,000 shares (the &quot;Target&quot;), the vesting of which is subject to the percentile of the Total Stock Return (&quot;TSR&quot;) of the Issuer's common stock over a three-year period, as determined by the Issuer's Board of Directors, versus the comparative TSRs of 12 peer public companies to be disclosed in the Issuer's 2019 Proxy Statement. Each PBRSU represents a contingent right to receive one share of the Issuer's common stock. The payout ratio of the Target, ranging from 0% to 200%, will depend on the degree of achievement of the percentile of the TSR.</footnote>
        <footnote id="F4">These PBRSUs are to vest in their entirety at the end of a three-year performance period as reported under footnote 3 above.</footnote>
        <footnote id="F5">As of the date of this filing, the total number of PBRSUs owned by the Reporting Person includes (a) a target number of 8,000 shares of PBRSUs granted on April 13, 2018, to vest in their entirety at the end of a three-year performance period; and (b) a target number of 8,000 shares of PBRSUs granted on April 12, 2019, to vest in their entirety at the end of a three-year performance period as reported under footnote 3 above.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ David Watson</signatureName>
        <signatureDate>2019-04-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
