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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-08-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001353283</issuerCik>
        <issuerName>SPLUNK INC</issuerName>
        <issuerTradingSymbol>SPLK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001252257</rptOwnerCik>
            <rptOwnerName>AUGUST CAPITAL MANAGEMENT III LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001252251</rptOwnerCik>
            <rptOwnerName>AUGUST CAPITAL III LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001114992</rptOwnerCik>
            <rptOwnerName>AUGUST CAPITAL STRATEGIC PARTNERS III LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001114719</rptOwnerCik>
            <rptOwnerName>AUGUST CAPITAL III FOUNDERS FUND LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001547399</rptOwnerCik>
            <rptOwnerName>August Capital Management V, L.L.C.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001452198</rptOwnerCik>
            <rptOwnerName>August Capital V, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001464232</rptOwnerCik>
            <rptOwnerName>August Capital Strategic Partners V, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001252521</rptOwnerCik>
            <rptOwnerName>JOHNSTON JOHN R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001193109</rptOwnerCik>
            <rptOwnerName>MARQUARDT DAVID F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001162157</rptOwnerCik>
            <rptOwnerName>RAPPAPORT ANDREW</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AUGUST CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>2480 SAND HILL ROAD, SUITE 101</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-08-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>510085</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.2613</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>13945602</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-08-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>681</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.2613</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18624</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares held of record by August Capital III, L.P. (&quot;August III&quot;), as nominee for August III (12,763,217 shares), August Capital Strategic Partners III, L.P. (232,613 shares)(&quot;August Strategic III&quot;), August Capital III Founders Fund (715,968 shares)(&quot;August III Founders&quot;) and related individuals. August Capital Management III, L.L.C. (&quot;ACM III&quot;), the general partner of each of August III, August Strategic III and August III Founders, may be deemed to have sole voting and investment power over the reported shares of the Issuer's common stock. John R. Johnston, David F. Marquardt and Andrew S. Rappaport, as members of ACM III, share voting and investment power over the reported shares of the Issuer's common stock.</footnote>
        <footnote id="F2">August III, August Strategic III, August III Founders and ACM III and each of its members disclaim beneficial ownership of these shares of Issuer's common stock except to the extent of any pecuniary interest therein, and the filing of this report is not an admission that August III, August Strategic III, August III Founders or ACM III or any of its members is the beneficial owner of these shares for purposes of Section 16 or for any other purpose.</footnote>
        <footnote id="F3">Shares held of record by August Capital V, L.P. (&quot;August V&quot;), as nominee for August V (18,336 shares), August Capital Strategic Partners V, L.P. (156 shares) (&quot;August Strategic V&quot;) and related individuals. August Capital Management V, L.L.C. (&quot;ACM V&quot;), the general partner of each of August V and August Strategic V, may be deemed to have sole voting and investment power over the reported shares of the Issuer's common stock.</footnote>
        <footnote id="F4">Howard Hartenbaum, David M. Hornik, John R. Johnston, David F. Marquardt, Vivek Mehra and Andrew S. Rappaport, as members of ACM V, share voting and investment power over the reported shares of the Issuer's common stock. August V, August Strategic V and ACM V and each of its members disclaim beneficial ownership of these shares of the Issuer's common stock except to the extent of any pecuniary interest therein, and the filing of this report is not an admission that August V, August Strategic V or ACM V or any of its members is the beneficial owner of these shares for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks>John R. Johnston, David F. Marquardt and Andrew S. Rappaport are the members of ACM III, which serves as the general partner to each of August III, August Strategic III and August III Founders. Howard Hartenbaum, David M. Hornik, John R. Johnston, David F. Marquardt, Vivek Mehra and Andrew S. Rappaport are the members of ACM V, which serves as the general partner to each of August V and August Strategic V. Each such person disclaims the existence of a &quot;group&quot; and disclaims beneficial ownership of any securities except to the extent of such person's pecuniary interest in such securities. *This report is one of two reports, each on a separate Form 4, but relating to the same transaction being filed by entities affiliated with August Capital and their applicable members.*</remarks>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management III, L.L.C.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management III, L.L.C., the General Partner of August Capital III, L.P.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management III, L.L.C., the General Partner of August Capital Strategic Partners III, L.P.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management III, L.L.C., the General Partner of August Capital III Founders Fund, L.P.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management V, L.L.C.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management V, L.L.C., the General Partner of August Capital V, L.P.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for August Capital Management V, L.L.C., the General Partner of August Capital Strategic Partners V, L.P.</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for John R. Johnston</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for David F. Marquardt</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Steven Simonian, by power of attorney for Andrew S. Rappaport</signatureName>
        <signatureDate>2012-08-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
