Luhn Algorithm Checker & Check Digit Calculator
Check a number with the Luhn (mod 10) algorithm or calculate its check digit, with a step-by-step table and the card network guessed from the prefix.
Passing the Luhn check only means the number has no simple typing error. It does not mean a card, account or device exists or is valid.
| Position | Digit | Doubled? | Value added |
|---|---|---|---|
| 11 | 7 | — | 7 |
| 10 | 9 | 9 × 2 = 18 → 9 | 9 |
| 9 | 9 | — | 9 |
| 8 | 2 | 2 × 2 = 4 | 4 |
| 7 | 7 | — | 7 |
| 6 | 3 | 3 × 2 = 6 | 6 |
| 5 | 9 | — | 9 |
| 4 | 8 | 8 × 2 = 16 → 7 | 7 |
| 3 | 7 | — | 7 |
| 2 | 1 | 1 × 2 = 2 | 2 |
| 1 | 3 | check digit | 3 |
The Luhn algorithm (mod 10) is the check digit behind payment card numbers, IMEI numbers and several national ID numbers. It catches every single-digit typo and most swaps of neighbouring digits. This tool checks a full number or works out the check digit to append, shows each doubling step, and guesses the card network from the first digits.
How to use this tool
- Choose whether you are checking a full number or calculating a check digit.
- Type or paste the digits; spaces and dashes are fine.
- Read the result and the corrected check digit if the number fails.
- Use the step table to follow the calculation by hand.
Formula
Starting from the rightmost digit of the payload, double every second digit (subtract 9 if the result is over 9) and add all digits. Check digit = (10 − (sum mod 10)) mod 10.
A full number is valid when the sum of all its digits, with the same doubling, is a multiple of 10. The check digit itself is never doubled.
Example
For 7992739871, doubling every second digit from the right gives the values 7, 9, 9, 4, 7, 6, 9, 7, 7, 2, which add up to 67. (10 − 7) mod 10 = 3, so the full number is 79927398713, and its Luhn sum is 70.
Frequently asked questions
Does a valid result mean the card is real?
No. Luhn only detects typing mistakes. Anyone can make a number that passes, and a real card can still be expired, blocked or fake. Never enter a real card number on a page you do not trust; this one runs entirely in your browser.
Which errors does Luhn miss?
It catches all single-digit errors and almost all swaps of two adjacent digits, except 09 ↔ 90. It cannot catch two errors that cancel out.
How is the card network detected?
From the issuer prefix: 4 is Visa, 51–55 and 2221–2720 are Mastercard, 34 and 37 are American Express, and so on. Prefix ranges change over time, so treat the guess as a hint.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/luhn-checksum-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). Luhn Algorithm Checker & Check Digit Calculator. https://kakao.mobile/tools/luhn-checksum-tool/
“Luhn Algorithm Checker & Check Digit Calculator.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/luhn-checksum-tool/.
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