Luhn Algorithm Checksum Validator
Validate a card number's Luhn (mod-10) checksum — the same algorithm every card network uses.
This checks the Luhn checksum only — it confirms the number is well-formed, not that it belongs to a real, active, or authorized account. Never enter a real card number here or anywhere outside a secure payment form.
Luhn checksum
Valid checksum
16 digits detected
For informational and educational purposes only — not professional or technical advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
How the mod-10 check digit works
Starting from the rightmost digit, every second digit is doubled — if doubling pushes it above 9, the digits of the result are summed (equivalent to subtracting 9). All digits are then added together, and the number passes if that total is a multiple of 10. This catches the most common data-entry errors — a single mistyped digit or two adjacent digits swapped — which is why every major card network appends a Luhn check digit, and why this is the standard first-line validation in checkout form testing.
Frequently asked questions
What is the Luhn algorithm actually checking?
It's a checksum formula that detects simple errors like a single mistyped digit or two adjacent digits swapped — it confirms a number is structurally well-formed, not that it corresponds to a real, active account.