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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2023-03-09</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001464963</issuerCik>
        <issuerName>First Eagle Alternative Capital BDC, Inc.</issuerName>
        <issuerTradingSymbol>FCRD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001802778</rptOwnerCik>
            <rptOwnerName>Mahmud Mehdi</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1345 AVENUE OF AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>48TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10105</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CEO of Adviser</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2023-03-09</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>37226</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On March 9, 2023, pursuant to an Agreement and Plan of Merger dated as of October 3, 2022 (the &quot;Merger Agreement&quot;), First Eagle Alternative Capital BDC, Inc., Crescent Capital BDC, Inc. (&quot;CCAP&quot;), Echelon Acquisition Sub, Inc., Echelon Acquisition Sub LLC and Crescent Cap Advisors, LLC (&quot;CCAP Advisor&quot;) effected a merger pursuant to which CCAP acquired all of the outstanding shares of FCRDs common stock in a multi-step stock and cash transaction (the &quot;Mergers&quot;). Pursuant to the Merger Agreement, upon completion of the Mergers, each issued and outstanding share of FCRD common stock was converted into the right to receive a pro rata portion, subject to FCRD stockholder elections and the adjustment mechanisms set forth in the Merger Agreement with respect to the CCAP Consideration (as defined below), of (1) approximately $8.6 million in cash payable by CCAP, (2) 6,174,383 validly issued, fully paid and</footnote>
        <footnote id="F2">continued from footnote 1: non-assessable shares of CCAPs common stock, par value $0.001 per share (together, the &quot;CCAP Consideration&quot;) and (3) $35 million in cash payable by CCAP Advisor. The disposition reported in this Form 4 is an exempt transaction.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Sabrina Rusnak-Carlson as Attorney-in-Fact</signatureName>
        <signatureDate>2023-03-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
