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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2019-12-16</periodOfReport>

    <dateOfOriginalSubmission>2019-12-17</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0000058492</issuerCik>
        <issuerName>LEGGETT &amp; PLATT INC</issuerName>
        <issuerTradingSymbol>LEG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001791961</rptOwnerCik>
            <rptOwnerName>CAMPBELL MARYELIZABETH R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>NO 1 LEGGETT ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CARTHAGE</rptOwnerCity>
            <rptOwnerState>MO</rptOwnerState>
            <rptOwnerZipCode>64836</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>51.18</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-12-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1832</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.24</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2020-12-31</value>
            </exerciseDate>
            <expirationDate>
                <value>2029-12-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1832</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1832</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects an option under the Issuer's Deferred Compensation Program exempt under Rule 16b-3. The Reporting Person elected to receive option shares in lieu of a portion of her 2020 director cash compensation. The Form 4 filed on 12/17/2019 reported the grant of 2,442 option shares in lieu of $25,000 of her foregone director cash payments for the first two quarters of 2020. Such option shares were believed to be fixed and determinable on the 12/16/2019 grant date (based upon the director cash compensation rates at that time), only conditioned upon the passage of time and continued directorship through her current term which expires at the next annual shareholder meeting expected to be held in May 2020. This Amendment reports a decrease of 610 option shares resulting from the reduction of her foregone director cash payment (from $12,500 to $6,250) for the second quarter of 2020 that otherwise would have been payable on 4/14/2020.  If the Reporting Person is re-elected as a director at the annual shareholder meeting, she will amend this Form 4 reporting additional shares granted under the option for the last two quarters in 2020.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ S. Scott Luton, attorney-in-fact</signatureName>
        <signatureDate>2020-04-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
