Luhn Algorithm Calculator

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Luhn (mod 10)scheme0values0valid0invalid0unreadable

The mod 10 algorithm behind card numbers, IMEIs, and many national IDs.

Open the full check digit validator with Luhn, IBAN, ISBN, GTIN, VIN, ISIN, Verhoeff, and Damm

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Luhn mod 10

The Luhn algorithm is the check digit behind payment card numbers, IMEI numbers, ISIN codes, and a long list of national identifiers. It was patented in 1954 and is deliberately trivial to compute by hand.

Validate mode reports the digit that was expected when a value fails, which is usually enough to spot a mistyped digit or a swapped pair.

Compute the check digit

Add check digit mode takes the payload and appends the final digit. 7992739871 becomes 79927398713.

Luhn is an error-detection scheme, not a security control. Anyone can generate a number that passes, so a passing value proves only that the digits are internally consistent.

Frequently Asked Questions

How does the Luhn algorithm work?

Starting from the rightmost digit, every second digit is doubled. Any doubled value above 9 has 9 subtracted from it. All digits are then summed, and the number passes if the total is divisible by 10.

What input does it accept?

Paste one number per line. Spaces and dashes are stripped, so grouped digits work as pasted.

Which errors does Luhn miss?

It catches every single-digit error and most adjacent transpositions, but it does not catch swapping 09 for 90. Verhoeff or Damm catch that case.