Luhn Algorithm Checker
Verify a Luhn (mod 10) checksum or compute a check digit, with a full step-by-step breakdown. Works for any Luhn-protected number.
- Instant results
- Nothing stored
- Runs in your browser
Instant results
Answers appear the moment you enter a number — no accounts, no setup, nothing to wait for.
Private by design
We never ask for or store a full card number — your sensitive details stay with you.
Free, no sign-up
Every tool is completely free to use — no account, no limits, no catch.
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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, you subtract 9 (equivalently, add the two digits). All the resulting values are summed. If the total is a multiple of 10, the number passes.
A worked example
Take 79927398713. Doubling alternate digits, adjusting values over 9, and summing gives 70 — a multiple of 10 — so it passes. Enter it above and expand Show the math to see each step.
Where Luhn is used
Beyond payment cards, the Luhn checksum protects IMEI device numbers, some government and loyalty IDs, and other identifiers. It is a lightweight guard against typos — not a security or authenticity check.
Luhn vs full validation
Passing Luhn is one of three checks the Card Validator performs — the others being character and network-length checks. Passing Luhn alone does not mean a number is a valid card.
Frequently asked questions
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