Luhn check digit validator
Check a Luhn digit string locally, guess the card brand, and show the expected check digit.
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Tool guide
Luhn check digit validator: how it works
Practical information to understand the result and use this tool correctly.
About Luhn check digit validator
Luhn doubles every second digit from the right and asks the sum to end in 0. Cards, some IMEI strings, and test numbers use it.
ISBN and IBAN already check their own digits. This stays on the Luhn rule, names Visa, Mastercard, or Amex when the prefix matches, and writes the expected check digit.
Letters, a single digit, or more than 19 digits stop before a false valid. The number never leaves the browser.
How to use it
- 1Paste a digit string. Spaces and dashes are ignored.
- 2Read valid or invalid, the brand guess, and the expected check digit.
- 3Copy the summary or reset to the Visa sample.
Frequently asked questions
- Is 4111 1111 1111 1111 valid?
- Yes. It is a 16-digit Visa test number. The Luhn sum ends in 0 and the check digit is 1.
- What does 4111 1111 1111 1112 show?
- Invalid. The expected check digit is 1, the typed check digit is 2, and the remainder is not 0.
- Does this charge a card or store the number?
- No. It only runs the checksum in the browser. It is not a payment form and it does not prove the account exists.