What is the Luhn algorithm?

The Luhn algorithm, also called the mod-10 algorithm, is a simple checksum used to catch accidental errors in identification numbers. Starting from the right, every second digit is doubled (subtracting 9 if over 9), all digits are summed, and the number is valid if the total is a multiple of 10.

Does a valid Luhn check mean the card is real?

No. The Luhn check only confirms the number is internally consistent and free of common typos. It says nothing about whether the card exists, is active, or has funds. Only the card network can confirm that.

Why does the tool show an expected check digit?

The check digit is the final digit, chosen so the whole number passes Luhn. If your number fails, the expected check digit tells you what that last digit should have been, which usually reveals a single mistyped or transposed digit.

Are the generated numbers real cards?

No. Generated numbers are fictitious — they only satisfy the Luhn rule and a brand prefix so you can test input validation and formatting. They are not issued to anyone and cannot be charged.

Is my card number sent anywhere?

No. Validation and generation happen entirely in your browser. Nothing is transmitted, which makes the tool safe to use even with a real card number you only want to syntax-check.

Luhn Algorithm Checker

The Luhn algorithm (mod-10) is the checksum that guards almost every payment card number, as well as IMEI device IDs and many national identifiers. It catches the most common data-entry mistakes — a single wrong digit or two swapped digits — before a number ever reaches a server. This tool runs that check locally and explains the result.

How it works

Working from the rightmost digit leftward:

Double every second digit.
If a doubled value is greater than 9, subtract 9 (equivalently, add its two digits).
Add up all the resulting values.
The number is valid when that total is a multiple of 10.

The final digit of the number is the check digit — it is specifically chosen so the whole sequence passes step 4. When a number fails, the tool computes what the check digit should have been, which usually pinpoints the typo.

It also inspects the leading digits to label the likely card brand (Visa starts with 4, Mastercard with 51-55 or 2221-2720, American Express with 34 or 37, and so on).

Tips and notes

Spaces and dashes are ignored, so you can paste 4539 1488 0343 6467 directly.
For example, 79927398713 is a classic Luhn-valid test value — its digits sum to a multiple of 10.
Use the generator to create fictitious, Luhn-valid numbers per brand for testing checkout forms. These are not real cards and cannot be charged; they only exercise your validation logic.
A pass means well-formed, not authorised. Always rely on your payment processor for real authorisation. Everything here stays on your device.