The last digit in your credit card number is a check digit derived from the other digits using the Luhn Formula as described in ISO/IEC 7812-1:1993. Its primary purpose is to ensure accurate entries of the credit card number during transactions. You can apply the same technique to other applications such as employee numbers or patient numbers. Using check digits for these numbers also ensures more accurate data entries:
Public Function CheckDigit(strNum As String) As Integer Dim i As Integer Dim iEven As Integer Dim iOdd As Integer Dim iTotal As Integer Dim strOneChar As String Dim iTemp As Integer ' Add digits in even ordinal positions ' starting from rightmost For i = Len(strNum) - 1 To 2 Step -2 strOneChar = Mid$(strNum, i, 1) If IsNumeric(strOneChar) Then iEven = iEven + CInt(strOneChar) End If Next i ' Process digits in odd ordinal positions ' starting from rightmost For i = Len(strNum) To 1 Step -2 strOneChar = Mid$(strNum, i, 1) If IsNumeric(strOneChar) Then ' Double it iTemp = CInt(strOneChar) * 2 If iTemp > 9 Then ' Break the digits (e.g., 19 becomes 1+9) iOdd = iOdd + (iTemp 10) + (iTemp - 10) Else iOdd = iOdd + iTemp End If End If Next i ' Add even and odd iTotal = iEven + iOdd ' Return the 10's complement CheckDigit = 10 - (iTotal Mod 10)End Function
To test, pass your credit card number, excluding the last digit, as a string parameter. The result should be the last digit of your credit card number.