<?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>0001358787</rptOwnerCik>
            <rptOwnerName>Deily Richard H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ARGAN, INC., ONE CHURCH</rptOwnerStreet1>
            <rptOwnerStreet2>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>Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <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>10000</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>10000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>40000</value>
                    <footnoteId id="F2"/>
                </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 10,000 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 purchase a total of 10,000 shares of common stock at the price of $33.85 per share with the date exercisable of 04/13/2017 and expiration date of 04/13/2026; (b) options to acquire 10,000 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 10,000 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 10,000  shares of common stock at a price of $50.30 per share, subject to the aforementioned three-year ratably vesting schedule, with Date Exercisable of 4/12/2020 and Expiration Date of 4/12/2029.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Richard H. Deily</signatureName>
        <signatureDate>2019-04-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
