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

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2010-09-28</periodOfReport>

    <noSecuritiesOwned>1</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001157408</issuerCik>
        <issuerName>K12 INC</issuerName>
        <issuerTradingSymbol>LRN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188921</rptOwnerCik>
            <rptOwnerName>BARRETT CRAIG R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5000 WEST CHANDLER BOULEVARD,</rptOwnerStreet1>
            <rptOwnerStreet2>MAILSTOP CH7-300</rptOwnerStreet2>
            <rptOwnerCity>CHANDLER</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85226</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <footnotes></footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Christopher R. Ryan, attorney-in-fact</signatureName>
        <signatureDate>2010-09-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
