Using Joel's algorithm function and a routine to loop through the 10
possible check digits, find the one that passes the algorithm and place it
in the adjacent column. Assuming your serial numbers are in column A,
starting at row 1, this will place the registration number in column B. The
actual starting point can be adjusted to fit in the code.

Sub AddCheckNum()
Dim LRow As Long, ctr As Integer
Dim TestNum As String
Dim rng As Range, c As Range

LRow = Cells(Rows.Count, 1).End(xlUp).Row 'change col num to suit
Set rng = Range("A1:A" & LRow) 'change col & start row to suit
For Each c In rng
For ctr = 0 To 9
TestNum = c.Value & ctr
If ValidateCardNumber(TestNum) Then
c.Offset(0, 1).Value = TestNum
Exit For
End If
Next
Next
End Sub


Public Function ValidateCardNumber(strCardNumber) As Boolean ' MOD 10
checkdigit. "Luhn algorithm"
On Error GoTo Err

Dim intLoop As Integer, intSum As Integer
Dim bIsAlternate As Boolean, intProduct As Integer

For intLoop = Len(strCardNumber) To 1 Step -1

If bIsAlternate = False Then
intSum = intSum + CInt(Mid(strCardNumber, intLoop, 1))
bIsAlternate = True
Else

intProduct = CInt(Mid(strCardNumber, intLoop, 1)) * 2

If Len(CStr(intProduct)) = 2 Then
intSum = intSum + CInt(Mid(intProduct, 1, 1)) +
CInt(Mid(intProduct, 2, 1))
Else
intSum = intSum + CInt(intProduct)
End If

bIsAlternate = False

End If

Next intLoop

If intSum Mod 10 = 0 Then
ValidateCardNumber = True
Else
ValidateCardNumber = False
End If

Exit Function

Err:
MsgBox "Error in ValidateCardNumber()" & vbCrLf & Err.Number &
Err.Description
End Function

Check for wordwrap in the VBA editor after you paste this into a standard
code module.
Mike F

"Abdul" wrote in message
...
Dear all,

I have the following serial numbers:

41078928128
41078928136
41078928144
41078928151
41078928169
41078928177
41078928185
41078928193
41078928201

The last digit is a check digit. How can I get the formula /algorithm
to get the check digit? So that if I have the serial numbers I can add
the check digit with it?

Thanks