<?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>3053</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>3053</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3053</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects an option under the Deferred Compensation Program exempt under Rule 16b-3. The Reporting Person elected to receive option shares in lieu of $25,000 of foregone director cash payments for the first two quarters of 2020, and a Form 4 was filed 12/17/2019 reporting the grant of 2,442 option shares. The Form 4 was amended on 4/16/2020 to report a decrease of 610 option shares (reflecting a new total of 1,832 option shares granted for the first two quarters of 2020) resulting from the reduction of her second quarter 2020 foregone cash payment (from $12,500 to $6,250). This amendment reports an additional 1,221 option shares granted in lieu of $12,500 of foregone director cash payments for the last two quarters in 2020. The additional option shares became fixed and determinable (only conditioned upon the passage of time, continued directorship and no further changes to her director cash payments in 2020) upon her election as a director at the 5/15/2020 annual shareholder meeting.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ S. Scott Luton, attorney-in-fact</signatureName>
        <signatureDate>2020-05-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
