Credit card check sums are based on the Luhn code. Double the odd digits (1, 3, 5, etc.). Use the sum of any 2-digit results. Add all these numbers together to end up with one single digit. Add this single digit to the sums of all the even digits. The Luhn check digit is the mod 10 of that final subtotal. For example 641205002340106 yields 4. jwhiting