🔢 Check Digit Calculator
Calculate the Luhn check digit for a number sequence, the same algorithm used to validate credit card numbers.
What is a check digit?
A check digit is an extra digit added to the end of a number sequence to help detect errors, such as a mistyped credit card number, bank account number, or ID code. The Luhn algorithm is one of the most widely used methods for calculating and validating check digits.
How the Luhn algorithm works
Starting from the rightmost digit and moving left, every second digit is doubled; if doubling produces a number greater than 9, subtract 9 from it. All digits (doubled and unchanged) are summed, and the check digit is the amount needed to make that sum a multiple of 10.
This calculator is for general educational purposes, such as learning how check-digit validation works. It is not a substitute for official card issuer or banking validation systems.