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Luhn Algorithm Step by Step

See exactly how the Luhn (mod 10) checksum is calculated for any number.

Tool inputs stay in your browser · optional AI search sends your query

Good to know: From the right, double every second digit; subtract 9 from results above 9; add everything; a valid number totals a multiple of 10.

How to use it (step by step)

  1. 1Type or paste your text into the “Number” box. There is already an example in it so you can see how it works; press Clear to start with your own.
  2. 2The answer appears right away in the “Working” box. No button needed.
  3. 3Press Copy and paste the result wherever you need it.

Words to know

Luhn check
A quick math test every real card number passes, like a spelling check for numbers. Passing it only means the number is typed in the right shape, not that the card is real.
Hash / fingerprint (SHA-256)
A short code calculated from a file or text. Change even one letter and the code becomes completely different, so it proves nothing was altered.

More in Cards & Payments

Luhn Check Digit CalculatorFind the last digit that makes a number pass the Luhn (mod 10) algorithm.Bulk Card Number ValidatorValidate a list of card numbers at once: network, length and Luhn, one per line.Card Brand DetectorIdentify the card network from the first digits: Visa, Mastercard, Amex, JCB, UnionPay and more.BIN / IIN Checker (Offline)Break down the first 6–8 digits of a card: network, industry (MII), lengths and IIN size.MII Lookup (First Card Digit)What the first digit of a card number means: banking, travel, airlines, petroleum…Card Expiry Date ValidatorCheck an MM/YY expiry: valid format, expired or not, and months left.