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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2010-02-01</periodOfReport>

    <dateOfOriginalSubmission>2010-02-03</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000787250</issuerCik>
        <issuerName>DPL INC</issuerName>
        <issuerTradingSymbol>dpl</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001305423</rptOwnerCik>
            <rptOwnerName>STEPHENSON GARY G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>DPL INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1065 WOODMAN DRIVE</rptOwnerStreet2>
            <rptOwnerCity>DAYTON</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>45432</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Executive VP, Operations</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-02-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>351</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>26.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>29458</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The total number of shares owned by the reporting person on a Form 4 filing made on August 3, 2007 inadvertently did not contain shares acquired through a dividend reinvestment plan.  Each subsequent Form 4 filing for the reporting person continued to inadvertently exclude shares acquired through this dividend reinvestment plan.  This amendment to the most recent Form 4 for the reporting person filed on February 3, 2010 is made to correct the total number of shares of common stock owned by the reporting person as of that Form 4's February 3, 2010 filing date by including the shares acquired through the dividend reinvestment plan and held as of that filing date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Joseph W. Mulpas by Power of Attorney</signatureName>
        <signatureDate>2010-03-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
