What is the Luhn algorithm?

The Luhn algorithm, also called mod 10, is a checksum formula that validates identification numbers such as credit cards and IMEIs. It doubles every second digit from the right, sums the digits, and a valid number has a total divisible by 10.

How is the Luhn check digit calculated?

Treat the payload as if a 0 check digit were appended, run the Luhn sum, then the check digit equals (10 - (sum mod 10)) mod 10. This is the single digit that makes the whole number pass.

Which numbers use Luhn?

Credit and debit card numbers (the final digit), IMEI device identifiers, some social insurance numbers and many other identifiers use Luhn as a built-in typo-detection check.

Does a valid Luhn result mean the card is real?

No. Luhn only confirms the digits are internally consistent, catching most typos and transpositions. It does not confirm the card was ever issued, is active, or has funds. Never treat it as authorization.

What errors does Luhn catch?

Luhn detects all single-digit errors and almost all adjacent transposition errors (swapping two neighbouring digits), except the specific swap of 0 and 9, which it misses.

Luhn Algorithm Checker

The Luhn algorithm (mod 10) is the check-digit formula behind credit card numbers, IMEI device identifiers and many national IDs. It exists to catch accidental typos and digit swaps before a number is processed. This checker validates any number and computes the correct check digit, entirely in your browser.

How it works

To validate a number that already includes its check digit:

Starting from the rightmost digit, double every second digit.
If a doubled value exceeds 9, subtract 9 (equivalently, add its two digits).
Sum all the resulting digits. The number is valid if the total is divisible by 10.

To compute the check digit for a payload that has no check digit yet, run the same weighting as if a 0 were appended, then the check digit is (10 - (sum mod 10)) mod 10.

Example

The number 4539 1488 0343 6467 produces a Luhn sum divisible by 10, so it passes. Change any single digit and the sum stops being a multiple of 10, which is exactly how Luhn catches typos.

Notes

Luhn detects all single-digit errors and nearly all adjacent transpositions, with the lone exception of swapping a 0 and a 9. A passing result means the digits are consistent, not that the card or device exists — it is a format check only. Nothing you type is sent anywhere.