Check Digit
Definition
- A check digit is a redundant digit or letter calculated from digits of a code number
- Added to the code number that permits accuracy of other digits in the code
Mod 11
- Assign weights: Start from the rightmost digit with weight 2, increasing by 1 moving left. (The check digit itself will later take weight 1.)
- Multiply & add: Multiply each digit by its weight and sum all products.
- Modulo 11: Divide the sum by 11 and find the remainder.
- Compute check digit: Subtract the remainder from 11.
- Handle special cases:
- If result = 11 → check digit = 0
- If result = 10 → check digit = X