Calculating the PostNet Barcode Checksum in Access Basic
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PostNet barcodes require a Modulo 10 check digit. This sample code accepts the zip code and calculates the check digit. It does not check for embedded dashes or other illegal characters, and it will process any length zip code.

Function Postnet_Checkdigit (InString As String) As Integer
    Dim Sum As Integer, i As Integer, CheckDigit As Integer

    Sum = 0
    For i = 1 To Len(InString)
        Sum = Sum + Val(Mid$(InString, i, 1))
    Next i
    CheckDigit = 10 - (Sum Mod 10)
    If CheckDigit = 10 Then
        CheckDigit = 0
    End If
    Postnet_Checkdigit = CheckDigit
End Function