Luhn Algorithm Validator: Check the Checksum of Any Number

The Luhn algorithm, also called the mod 10 checksum, is the formula behind the last digit of every credit and debit card number in circulation. It exists to catch a simple, common mistake, a single mistyped digit or two digits swapped, before a number is even sent anywhere for real verification. This tool runs the exact same math so you can confirm any number is internally consistent, whether you are debugging a checkout form or double-checking an imported dataset.

What the Luhn algorithm actually checks

Starting from the rightmost digit, which is the check digit itself, and moving left, every second digit gets doubled. If doubling produces a two-digit result, its digits are added together, which is mathematically the same as subtracting 9. Every digit, doubled or not, is then summed into a single total, and the number passes only if that total is an exact multiple of 10. It is entirely arithmetic, no external data or lookup is involved.

Where the Luhn checksum is used

Card numbers under the ISO/IEC 7812 standard are the most common example, but the same formula shows up in IMEI numbers that uniquely identify mobile phones, Canadian Social Insurance Numbers, US National Provider Identifier numbers used in healthcare billing, and assorted loyalty and membership card formats that want a cheap way to catch typos.

What it cannot tell you

A valid Luhn checksum only proves the digits are arranged in a way the formula accepts, it says nothing about whether a card is active, funded, or has ever been issued to anyone. Real payment validation happens through the card network and the issuing bank, this tool is meant for developers testing form logic and data integrity, not for verifying real payment credentials.