Luhn Checksum Validator
Check whether a digit sequence passes the Luhn (mod 10) checksum, or compute the check digit for a partial sequence.
About this tool
The Luhn algorithm is a simple checksum used to catch accidental errors (like a mistyped or transposed digit) in identifier numbers — most commonly credit/debit card numbers, but also IMEI numbers for mobile devices and various other identifiers. It works by doubling every second digit from the right, subtracting 9 from any result over 9, summing all the digits, and checking that the total is a multiple of 10. It's a basic error-detection check, not a security or fraud-prevention measure, and it cannot tell you whether a card or identifier actually exists or is in use. Never enter a real card number into any online tool, including this one — it's intended purely for exploring the algorithm itself.