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

    <schemaVersion>X0204</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2011-08-10</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001340127</issuerCik>
        <issuerName>Carbonite Inc</issuerName>
        <issuerTradingSymbol>CARB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001104329</rptOwnerCik>
            <rptOwnerName>CROSSLINK CAPITAL INC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO EMBARCADERO CENTER</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2200</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94111</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Affiliate of Director</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Seres D Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>988578</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Notes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting persons are Crosslink Capital, Inc. (&quot;Crosslink&quot;) and Michael J. Stark.  The securities reported as beneficially owned by Crosslink include securities also beneficially owned by Crosslink Ventures V Holdings, L.L.C. (&quot;Ventures V Holdings&quot;), a Delaware limited liability company, Crossover Fund V Management, L.L.C., a Delaware limited liability company (&quot;Crossover V Management&quot;) and Mr. Stark (collectively, the &quot;Crosslink Affiliates&quot;).  Crosslink is an investment adviser to investment funds of which Ventures V Holdings or Crossover V Management is the general partner, manager or holder of Class B Units.  Mr. Stark is the control person of Crosslink, Ventures V Holdings and Crossover V Management.  Gary Hromadko, an affiliate of Crosslink, is a director of the Issuer and is the representative of Crosslink and the Crosslink Affiliates.</footnote>
        <footnote id="F2">Crosslink and the Crosslink Affiliates disclaim membership in a group within the meaning of Rule 13d-5(b) under the Securities Exchange Act of 1934, as amended.  These securities are held directly by investment funds to which Crosslink is investment adviser for the benefit of the investors in those funds.  These securities are indirectly beneficially owned by Crosslink in such capacity as investment adviser, by Ventures V Holdings and Crossover V Management as the general partner, manager or holder of Class B Units of one or more of those funds, and by Mr. Stark as the control person of those entities. Crosslink and the Crosslink Affiliates disclaim beneficial ownership of such securities except to the extent of their respective pecuniary interest therein.</footnote>
        <footnote id="F3">Each share of Series D Preferred Stock will automatically convert into three shares of the Issuer's common stock on the completion of the Issuer's initial public offering and has no expiration date.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Crosslink Capital, Inc., by Jerome S. Contro, Chief Operating Officer</signatureName>
        <signatureDate>2011-08-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
