Luhn Checksum Checker

How the Luhn algorithm works

From the rightmost digit, double every second digit; if doubling gives a two-digit number, subtract 9. Sum everything — a valid number's sum is divisible by 10. It catches every single-digit typo and most adjacent swaps, which is why card forms can reject a mistyped number before contacting anyone.

Frequently asked questions

Does passing the Luhn check mean a card is real?

No — it only means the digits are self-consistent. Whether an account exists, has funds, or is yours is decided by the issuing bank during authorization. This tool is for debugging payment forms and validating data entry, not verifying cards.

Is it safe to paste a card number here?

The check runs entirely in your browser with no network requests. For development, prefer the official test numbers (like 4111 1111 1111 1111) that payment providers publish — they pass Luhn but can never charge anyone.

What else uses Luhn?

Phone IMEIs (dial *#06# and the 15th digit is a Luhn check), Canadian Social Insurance Numbers, and many gift card and loyalty schemes.