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

    <schemaVersion>X0204</schemaVersion>

    <documentType>3/A</documentType>

    <periodOfReport>2011-07-19</periodOfReport>

    <dateOfOriginalSubmission>2011-07-19</dateOfOriginalSubmission>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001334814</issuerCik>
        <issuerName>ZILLOW INC</issuerName>
        <issuerTradingSymbol>Z</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001082906</rptOwnerCik>
            <rptOwnerName>HOAG JAY C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001197937</rptOwnerCik>
            <rptOwnerName>KIMBALL RICK</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188456</rptOwnerCik>
            <rptOwnerName>DREW JOHN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188444</rptOwnerCik>
            <rptOwnerName>REYNOLDS JON Q JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283643</rptOwnerCik>
            <rptOwnerName>TECHNOLOGY CROSSOVER MANAGEMENT V LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001274664</rptOwnerCik>
            <rptOwnerName>TCV V LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283642</rptOwnerCik>
            <rptOwnerName>TCV MEMBER FUND L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Non-Qualified Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.5234</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-02-15</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-02-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3327</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Non-Qualified Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.5234</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2008-02-15</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-02-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1110</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">These stock options were omitted from the Reporting Person's original Form 3.</footnote>
        <footnote id="F2">These stock options are directly held by Jay C. Hoag. Mr. Hoag has sole dispositive power over the stock options and the underlying shares; however, TCV Management 2004, L.L.C. (&quot;TCM 2004&quot;) owns 100% of the pecuniary interest therein. Mr. Hoag, Richard H. Kimball, John L. Drew, and Jon Q. Reynolds, Jr. are Members of TCM 2004, but each disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for Jay C. Hoag</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for Richard H. Kimball</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for John L. Drew</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for Jon Q. Reynolds, Jr.</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for Technology Crossover Management V, L.L.C.</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for TCV V, L.P.</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Frederic D. Fenton, Authorized signatory for TCV Member Fund, L.P.</signatureName>
        <signatureDate>2012-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
