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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-02-11</periodOfReport>

    <notSubjectToSection16>false</notSubjectToSection16>

    <issuer>
        <issuerCik>0001126956</issuerCik>
        <issuerName>SPIRE INC</issuerName>
        <issuerTradingSymbol>SR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001526310</rptOwnerCik>
            <rptOwnerName>Sitherwood Suzanne</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>700 MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ST. LOUIS</rptOwnerCity>
            <rptOwnerState>MO</rptOwnerState>
            <rptOwnerZipCode>63101</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>true</isDirector>
            <isOfficer>true</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle>President &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-02-11</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>false</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>119741.00</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>119741.00</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Irrevocable trust</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">It has been determined that Ms. Sitherwood's revocable trust owns two fewer shares of Spire stock than previously reported. The trust owns 119,741 shares. The two-share difference arises from a Form 4/A filed on October 4, 2013 that should have reported the vesting of 6,665 shares, but incorrectly reported 6,667.</footnote>
        <footnote id="F2">This transaction represents the transfer of shares from a revocable trust of which Ms. Sitherwood is the sole trustee to an irrevocable trust of which Mr. Carl Bouckhaert (Ms. Sitherwood's partner) and Mr. Marc Dearth (no relation to Ms. Sitherwood) are co-trustees. The beneficiaries of the irrevocable trust are Mr. Bouckaert and Ms. Sitherwood's heirs. Ms. Sitherwood has limited dispositive power over the shares and no voting power over the shares.</footnote>
        <footnote id="F3">Mr. Carl Bouckhaert (Ms. Sitherwood's partner) and Mr. Marc Dearth (no relation to Ms. Sitherwood) are co-trustees of this irrevocable trust. The beneficiaries of the trust are Mr. Bouckaert and Ms. Sitherwood's heirs. Ms. Sitherwood has limited dispositive power over the shares and no voting power over the share</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Ellen Theroff, Attorney-in-Fact for Suzanne Sitherwood</signatureName>
        <signatureDate>2022-02-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
