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

    <schemaVersion>X0205</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2012-03-18</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001170010</issuerCik>
        <issuerName>CARMAX INC</issuerName>
        <issuerTradingSymbol>KMX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001545342</rptOwnerCik>
            <rptOwnerName>Nash William D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>12800 TUCKAHOE CREEK PARKWAY</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>RICHMOND</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>23238</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, HR &amp; Admin Services</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>126</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.19</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-06-24</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-06-24</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.19</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-06-24</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-06-24</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>24.99</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2008-04-03</value>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2014-04-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>30800</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>24.99</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2008-04-03</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2014-04-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>30800</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>19.82</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2009-04-07</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-04-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>48000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>19.82</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2009-04-07</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-04-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>48000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.43</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-04-07</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2016-04-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>39656</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.43</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-04-07</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2016-04-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>39656</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>25.39</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-04-06</value>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-04-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>64894</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>25.39</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-04-06</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-04-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>64894</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>32.69</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-04-05</value>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-04-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>64894</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>32.69</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-04-05</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-04-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>64894</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>30.24</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-12-27</value>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-12-27</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14952</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>SARS</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>30.24</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-12-27</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-12-27</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14952</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F9"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F9"/>
                <footnoteId id="F10"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4451</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F11"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F10"/>
                <footnoteId id="F11"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6482</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F12"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F12"/>
                <footnoteId id="F13"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6482</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F14"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F13"/>
                <footnoteId id="F15"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1539</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The stock options vested in four equal installments on each of June 24, 2006, June 24, 2007, June 24, 2008 and June 24, 2009 and are now fully exercisable.</footnote>
        <footnote id="F2">The stock options and the stock appreciation rights (SARS) were granted in tandem. Accordingly, the exercise of one results in the surrender to the Issuer of the other. The SARS, which will entitle the Reporting Person to receive the cash value of the options in lieu of exercising the options, become exercisable only following a change in control of the Issuer as set forth in the Issuer's 2002 Stock Incentive Plan, as amended and restated.</footnote>
        <footnote id="F3">The stock options vested in four equal installments on each of April 3, 2008, April 3, 2009, April 3, 2010 and April 3, 2011 and are now fully exercisable.</footnote>
        <footnote id="F4">The stock options are exercisable with respect to one-fourth of the underlying shares of Common Stock on each of April 7, 2009, April 7, 2010, April 7, 2011 and April 7, 2012.</footnote>
        <footnote id="F5">The stock options are exercisable with respect to one-fourth of the underlying shares of Common Stock on each of April 7, 2010, April 7, 2011, April 7, 2012 and April 7, 2013.</footnote>
        <footnote id="F6">The stock options are exercisable with respect to one-fourth of the underlying shares of Common Stock on each of April 6, 2011, April 6, 2012, April 6, 2013 and April 6, 2014.</footnote>
        <footnote id="F7">The stock options are exercisable with respect to one-fourth of the underlying shares of Common Stock on each of April 5, 2012, April 5, 2013, April 5, 2014 and April 5, 2015.</footnote>
        <footnote id="F8">The stock options are exercisable with respect to one-fourth of the underlying shares of Common Stock on each of December 27, 2012, December 27, 2013, December 27, 2014 and December 27, 2015.</footnote>
        <footnote id="F9">The restricted stock units shall vest on April 7, 2012.</footnote>
        <footnote id="F10">Shares of Company common stock will be issued to the Reporting Person following vesting of the restricted stock units, which are referred to by the Company as market stock units (the &quot;MSUs&quot;), in accordance with the terms of the Form of Notice of Market Stock Unit Grant filed as Exhibit 10.2 to the Company's Current Report on Form 8-K filed on April 2, 2009. The minimum number of shares of Company common stock that will be issued to the Reporting Person at payment is zero, and the maximum number of shares of Company common stock that will be issued at payment is two times the number of MSUs.</footnote>
        <footnote id="F11">The restricted stock units shall vest on April 6, 2013.</footnote>
        <footnote id="F12">The restricted stock units shall vest on April 5, 2014.</footnote>
        <footnote id="F13">Shares of Company common stock will be issued to the Reporting Person following vesting of the restricted stock units, which are referred to by the Company as market stock units (the &quot;MSUs&quot;), in accordance with the terms of the Form of Notice of Market Stock Unit Grant filed as Exhibit 10.2 to the Company's Current Report on Form 8-K filed on October 22, 2010. The minimum number of shares of Company common stock that will be issued to the Reporting Person at payment is zero, and the maximum number of shares of Company common stock that will be issued at payment is two times the number of MSUs.</footnote>
        <footnote id="F14">The restricted stock units shall vest on December 27, 2014.</footnote>
        <footnote id="F15">Shares of Company common stock will be issued to the Reporting Person following vesting of the restricted stock units, which are referred to by the Company as market stock units (MSUs), in accordance with the terms of the Form of Notice of Market Stock Unit Grant filed as Exhibit 10.2 to the Company's Current Report on Form 8-K filed on December 23, 2011. The minimum number of shares of Company common stock that will be issued to the Reporting Person at payment is zero, and the maximum number of shares of Company common stock that will be issued at payment is two times the number of MSUs.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Heather Saunders</signatureName>
        <signatureDate>2012-03-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
