Luhn Checksum Validator
How it works
The Luhn algorithm (also called "modulus 10" or "mod 10") is a simple checksum formula used to validate identification numbers — primarily credit card numbers, but also IMEI numbers (mobile phone identifiers), Canadian Social Insurance Numbers, and several other ID formats. It catches single-digit transcription errors (a digit being entered wrongly) and some transposition errors (two adjacent digits being swapped).
**The algorithm** 1. From the rightmost digit (check digit), moving left, double the value of every second digit. If doubling produces a number > 9, subtract 9. 2. Sum all digits (doubled or original). 3. If the total is divisible by 10, the number is valid.
Example: 4532015112830366. Working right to left, double every other digit... sum = 40. 40 mod 10 = 0 → valid.
**What Luhn validation tells you** A Luhn-valid number is NOT necessarily a real, active credit card number — it just passes the checksum. Luhn validation catches: single-digit errors (wrong digit typed), some adjacent transpositions. It doesn't catch: two non-adjacent transpositions, many multi-digit errors. It is not a security measure — it's an error-detection code.
**Credit card number structure** The first 6 digits are the Issuer Identification Number (IIN/BIN): 4 = Visa, 51–55 = Mastercard, 34/37 = Amex, 6011 = Discover. Digits 7 to (n-1) are the account number. The last digit is the Luhn check digit.
**Real-world use** Payment forms use Luhn validation as a client-side pre-check before submitting to a payment processor — catching typos immediately without a network round-trip. This reduces fraud attempts and form submission errors simultaneously.
Privacy: card numbers are validated locally in the browser. No data is transmitted.
Frequently Asked Questions
- No. Luhn validation only confirms the number passes the checksum formula — it doesn't verify the number belongs to any real account, that the account is open, or that there are funds available. Randomly generated numbers that pass Luhn are not valid credit cards and will be rejected when submitted to a payment processor. The Luhn check is a client-side pre-validation to catch typos before network submission, not a security measure or account verification tool.
- The Luhn algorithm catches: all single-digit substitution errors (any one digit is wrong). It catches most (but not all) adjacent transposition errors (two neighbouring digits are swapped — Luhn misses transpositions of 0 and 9, since they produce the same contribution). It does NOT catch: all twin errors (the same digit substituted for another at two positions); many multi-digit errors; or non-adjacent digit transpositions. Detection rate: approximately 100% of single errors, ~90% of two-digit errors.
- To generate the check digit for a number: (1) drop or leave as 0 the last digit. (2) Apply the Luhn algorithm to get a sum S. (3) Check digit = (10 − (S mod 10)) mod 10. Example: for a 15-digit Amex number, apply the algorithm to the first 14 digits, then calculate the 15th digit so the full 15-digit number sums to a multiple of 10. Payment processors use this to generate valid test card numbers for sandbox environments.
- Beyond credit cards, Luhn is used for: IMEI numbers (15-digit mobile phone hardware identifiers — the 15th digit is a Luhn check). Canadian Social Insurance Numbers (SIN — 9 digits, last is Luhn check). Israeli identity card numbers. South African ID numbers. National Provider Identifier (NPI) numbers in US healthcare. Some ISBNs (though ISBN-10 uses a modulo-11 algorithm, not Luhn). The algorithm is also used in some barcode validation schemes and financial account number validation.