Calculate Check Digits

Tip: We recommend that you use a check digit to ensure finder numbers are entered accurately during gift entry.

If you use a check digit with your finder numbers, the program uses the Luhn, or mod 10, algorithm to validate finder numbers. The algorithm detects single-digit errors as well as transpositions of adjacent digits. To generate the validation calculation, the program appends a single digit to each finder number.

To calculate a check digit, the program uses this algorithm:

1. The program starts with the last digit in the finder number and, as it moves to the left, doubles the value of every other digit. For example, to calculate the check digit for finder number 56789:

   9	9 * 2 = 18	18
   8
   7	7 * 2 = 14	14
   6
   5	5 * 2 = 10	10

2. The program adds the individual digits of the doubled numbers to the undoubled numbers from the original finder number.

   9	9 * 2 = 18	18	1 + 8 = 9	9
   8				8
   7	7 * 2 = 14	14	1 + 4 = 5	5
   6				6
   5	5 * 2 = 10	10	1 + 0 = 1	1
   				9 + 8 + 5 + 6 + 1 = 29

   Note: The modulo operation finds the remainder when you divide a number. X modulo Z (abbreviated as X mod Z) is the remainder when you divide X by Z. For example, “7 mod 3” equals 1, while “9 mod 3” equals 0.

3. The program calculates the sum of the digits mod 10. For this example, 29 mod 10 = 9. (29 divided by 10 = 2 with a remainder of 9.)

4. For the check digit calculation, the program subtracts the remainder from 10. For example, 10 - 9 = 1. The check digit equals 1.

In our example, the full “valid” finder number (including check digit) is 567891. If the final value in the finder number was any number other than 1, then the finder number would be “invalid” or corrupt. Numbers like 567892, 567893, 567894, 567895, 567896, 567897, 567898, 567899, and 567890 are all “invalid” or corrupt finder numbers because 1 is the only valid check digit number for finder number 56789.