Luhn Validator (Mod 10 Checksum)

The Luhn algorithm, also called the mod 10 check, is a simple checksum formula. Starting from the rightmost digit, it doubles every second digit, subtracts nine from any result over nine, sums all the digits, and the number is valid when that total is a multiple of ten. It catches most single-digit typos and digit transpositions.

Lots of identifiers do: credit and debit card numbers, IMEI device serials, some national provider and tax identifiers, and many loyalty or account numbers. This generic tool runs the raw checksum on any digit string, so it works regardless of what the number represents.

Enter the number without its final digit, and the tool shows the digit that should go in the last position to make the whole string pass the Luhn check. Append that digit and the number will validate. This is exactly how systems create the check digit when issuing a new number.

No. The Luhn check only proves a number is internally consistent. It cannot tell you whether a card, device, or account actually exists or is active. It is a typo-catching tool, not a security or lookup feature.

No. All validation runs locally in your browser using client-side JavaScript. The number you enter is never uploaded, logged, or stored, so it never leaves your device.