Tech
Luhn Checksum Calculator
Validate any credit card, IMEI, or Luhn-checksum identifier, or generate the check digit for a partial number. Shows the step-by-step mod 10 trace, detects Visa, Mastercard, Amex, and Discover BINs, and flags trivially valid numbers.
Input
Spaces and dashes are stripped automatically. Up to 30 digits.
Quick reference
Luhn (mod 10) is a checksum that catches single-digit typos and most adjacent transpositions in identifiers like credit cards and IMEI numbers.
It is a format check only. A valid Luhn number can still be made up, expired, or unauthorized. Never treat Luhn validity as proof that a card or device is real or active.
Result
Luhn valid
4111 1111 1111 1111
Normalized digits4111111111111111
Digit count16
Detected card typeVisa
All same digitNo
Step-by-step Luhn trace
| Pos. (right) | Digit | Doubled? | After x2 | Adds |
|---|---|---|---|---|
| 16 | 4 | Yes | 8 | 8 |
| 15 | 1 | No | 1 | |
| 14 | 1 | Yes | 2 | 2 |
| 13 | 1 | No | 1 | |
| 12 | 1 | Yes | 2 | 2 |
| 11 | 1 | No | 1 | |
| 10 | 1 | Yes | 2 | 2 |
| 9 | 1 | No | 1 | |
| 8 | 1 | Yes | 2 | 2 |
| 7 | 1 | No | 1 | |
| 6 | 1 | Yes | 2 | 2 |
| 5 | 1 | No | 1 | |
| 4 | 1 | Yes | 2 | 2 |
| 3 | 1 | No | 1 | |
| 2 | 1 | Yes | 2 | 2 |
| 1 | 1 | No | 1 |
Sum = 30 and 30 mod 10 = 0. A Luhn-valid number has a sum that is divisible by 10.
Frequently Asked Questions about the Luhn Checksum Calculator
What is the Luhn algorithm?
The Luhn algorithm (also called mod 10 or modulus 10) is a simple checksum formula invented by IBM scientist Hans Peter Luhn in 1954 and patented in 1960 (US Patent 2,950,048). The patent expired and the algorithm entered the public domain in 1977, which is why it shows up in so many open standards. From the rightmost digit moving left, every second digit is doubled (and 9 subtracted if the result exceeds 9), then all the digits are summed. The number is valid if the total is divisible by 10.
What errors does Luhn catch and what does it miss?
Luhn catches every single-digit typo and most adjacent-digit transpositions, which together account for the vast majority of human keying errors. It does not catch the specific transposition of 09 to 90 or 90 to 09 (both pairs sum to the same Luhn contribution), nor twin-digit substitutions like 22 swapped with 55. It also will not detect insertions or deletions that change the digit count. Luhn is a cheap, lightweight checksum, not a cryptographic hash.
Where is the Luhn algorithm actually used?
Most magnetic-stripe credit and debit card numbers carry a Luhn check digit (Visa, Mastercard, Amex, Discover, Diners, JCB). It also appears in IMEI device identifiers, the Canadian Social Insurance Number (SIN), the US National Provider Identifier (NPI) for healthcare, some national identifiers (Greek SSN, Israeli ID), survey IDs, and many internal account-number schemes. Anywhere a system needs a quick format check on a hand-keyed identifier, Luhn is the default.
Does a valid Luhn number mean the card is real or authorized?
No. Luhn is a format check only. A made-up number like 4111 1111 1111 1111 (Visa's documented test card) passes Luhn but is not a real account. Authorization, fraud detection, balance, and expiration are all separate checks the card network and issuer perform after Luhn rejects the obvious typos. Treat Luhn as a typo filter to give users instant feedback on a form, not as proof a number is genuine.
Why does the calculator flag numbers like 0000000000000000?
A string of identical digits where the doubling pattern works out to a multiple of 10 will pass Luhn trivially, even though no issuer would ever assign such a number. The flag on all-same-digit inputs is there so you do not mistakenly conclude that a placeholder, test value, or empty form field is a valid identifier. The check digit is doing its job mathematically; the warning is doing its job semantically.