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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-12-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001463638</rptOwnerCik>
            <rptOwnerName>Flanders Cynthia</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>10006 WILDWOOD ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>KENSINGTON</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>20895</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>15000</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>40.15</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-12-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2020-12-23</value>
            </exerciseDate>
            <expirationDate>
                <value>2029-12-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>67000</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On December 23, 2019 (the &quot;Grant Date&quot;), the Reporting Person received 10-year options, vesting ratably over three years on each anniversary date from the Grant Date, to purchase 10,000 shares of the Issuer's common stock with an exercise price of $40.15 per share.</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 7,000 shares of common stock at a price of $26.88 per share with Date Exercisable of 12/17/2014 and Expiration Date of 12/17/2023; (b) options to acquire 10,000 shares of common stock at a price of $31.82 with Date Exercisable of 12/11/2015 and Expiration Date of 12/11/2024; (c) options to acquire 10,000 shares of common stock at a price of $30.44 per share with Date Exercisable of 1/7/2017 and Expiration Date of 1/7/2026; (d) options to acquire 10,000 shares of common stock at a price of $71.75 per share with Date Exercisable of 1/5/2018 and Expiration Date of 1/5/2027; (e) options to acquire 10,000 shares of common stock at a price of $46.35 per share with initial Date Exercisable of 1/11/2019 and Expiration Date of 1/11/2028;</footnote>
        <footnote id="F3">(f) options to acquire 10,000 shares of common stock at a price of $41.19 per share with initial Date Exercisable beginning on 12/12/2019 and Expiration Date of 12/12/2028; and (g) options to acquire 10,000 shares of common stock at a price of $40.15 per share with initial Date Exercisable of 12/23/2020 and Expiration Date of 12/23/2029.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Cynthia A Flanders</signatureName>
        <signatureDate>2019-12-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
