What is the Luhn algorithm used for?

The Luhn (mod-10) algorithm is a checksum used to catch typos in credit card numbers, IMEI numbers, SIM ICCIDs and many national identification numbers. It does not prove the number is real or active — only that it is internally consistent.

How do you calculate a Luhn check digit?

Append a 0 to your payload, run the Luhn sum, then the check digit is (10 − (sum mod 10)) mod 10. This tool shows that digit for whatever you type, so you can build valid test numbers.

Does passing the Luhn check mean a card is valid?

No. Passing only means the digits are mathematically consistent. It cannot confirm the card exists, is active, or belongs to anyone. It is purely a typo-detection check.

Which errors does the Luhn algorithm catch?

It detects all single-digit errors and almost all transpositions of two adjacent digits. The one transposition it misses is swapping 09 with 90, because both produce the same doubled value.

What is the difference between Luhn (mod-10) and mod-97?

Luhn is a mod-10 checksum used for card and device numbers, doubling every second digit. Mod-97 is a different, stronger scheme used for IBANs and some VAT numbers, dividing the whole number by 97. They are not interchangeable.

Luhn Algorithm Checker

Q: Is my number sent anywhere?

No — the entire Luhn calculation runs in your browser. Nothing you type is uploaded, logged, or stored.

Luhn algorithm (mod-10) checker

The Luhn algorithm is a simple checksum invented by Hans Peter Luhn and used to catch accidental typos in identification numbers — credit and debit cards, IMEI device serials, SIM ICCIDs, and many national IDs all carry a Luhn check digit. This tool tells you instantly whether a number passes, and also computes the check digit, which is handy for generating consistent test data.

How it works

Walking the digits right to left:

Every second digit is doubled. If doubling gives a result over 9, subtract 9 (equivalent to adding the two digits).
All the digits are summed.
The number is valid when the total is a multiple of 10.

To find the check digit for a payload (a number without its final digit), the tool computes the Luhn sum as if a 0 were appended, then the answer is (10 − sum mod 10) mod 10.

Example

Check 4539 1488 0343 6467 (a Luhn-valid test card):

Doubling every second digit from the right and reducing values over 9 produces a running total.
That total is a multiple of 10 → passes the Luhn check.

Now take a payload 7992739871 with no check digit. The Luhn sum of 79927398710 leaves a remainder, and the tool reports the required check digit as 3, so 79927398713 would pass.

What Luhn protects (and doesn’t)

Detects	Misses
Any single wrong digit	The 09 ↔ 90 transposition
Most adjacent transpositions	Whether the account is real
Most twin-digit errors	Whether the card is active

Privacy: the entire calculation runs in your browser. Nothing you type is uploaded, logged, or stored. A pass means the number is internally consistent — never that it belongs to a real, active account.