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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-08-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001069533</issuerCik>
        <issuerName>RGC RESOURCES INC</issuerName>
        <issuerTradingSymbol>RGCO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001282618</rptOwnerCik>
            <rptOwnerName>SMOOT RAYMOND D JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1447 LUSTERS GATE ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>BLACKSBURG</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>24060</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-08-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>86.407</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>35298.751</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Purchase made with optional cash contribution pursuant to the RGC Resources, Inc. Dividend Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F2">34,923.751 shares owned directly by insider. Includes 109.428 restricted stock shares purchased through dividends reinvested in the Restricted Stock Plan for Outside Directors of RGC Resources, Inc. and 17.749 unrestricted shares purchased through dividends reinvested in the Dividend Reinvestment and Stock Purchase Plan of RGC Resources, Inc.
375 shares owned indirectly by spouse.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Raymond D. Smoot, Jr. By Howard T. Lyon, POA dated 01/24/2005</signatureName>
        <signatureDate>2017-08-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
