Credit Card Number Validator (Luhn)

How the Luhn algorithm works

Take 4111 1111 1111 1111. Working from the right:

1. Double every second digit.
2. If doubling gives more than 9, subtract 9 (so 14 becomes 5).
3. Add every digit together.
4. The number is well formed if the total is a multiple of ten.

The final digit of a card number is the check digit — chosen precisely so that the total comes out right.

What it catches, and what it does not

It catches every single-digit error and almost every transposition of adjacent digits, which between them are the overwhelming majority of typing mistakes. It does not catch a transposition of 09 and 90, and it obviously cannot tell whether a well-formed number was ever issued.

Please use test numbers

Every payment provider publishes card numbers for testing — 4111 1111 1111 1111 for Visa, 5555 5555 5555 4444 for Mastercard, 3782 822463 10005 for American Express. They pass the Luhn check and belong to nobody.

This page performs its check locally and makes no network request. That is worth stating, and it is also worth saying plainly that you have no way to confirm it from the outside — which is exactly why you should never type a real card number into any site other than a payment form you trust.