Credit Card Validator: How the Luhn Check Works

Ever mistyped a card number and had a form reject it instantly, before it ever reached the bank? That's the Luhn check at work. Here's how credit card validation works — and its limits.

Validate a card number's format

Paste a number into the credit card validator to check whether it passes the Luhn checksum and to identify the likely card brand from its opening digits. It never contacts any bank — it only checks the math.

How the Luhn algorithm works

Starting from the right, every second digit is doubled (and 9 subtracted if the result exceeds 9), then all digits are summed. If the total is divisible by 10, the number passes. It's a simple checksum designed to catch typos and single-digit errors.

What it proves — and doesn't

Passing Luhn means the number is well-formed, not that it's a real, active, funded card. It catches mistyped digits and obviously fake numbers, but only the payment processor can confirm a card actually exists and has funds.

Why it exists

The check lets forms and systems reject typos immediately, saving a pointless round-trip to the bank. It's used on credit cards, some ID numbers and IMEI codes.

A note on safety

Never paste real card numbers into random websites. This tool runs entirely in your browser and contacts nothing — but as a habit, only validate test numbers.