Luhn Algorithm Calculator (Check Digit & Validator)

The Luhn algorithm (also called the "mod 10" algorithm) is the checksum formula behind credit and debit card numbers, IMEI numbers, and several other identifiers — it exists to catch accidental typos and transposed digits, not to verify an account is real. Starting from the rightmost digit and moving left, every second digit is doubled; if doubling produces a number above 9, its digits are summed (equivalently, 9 is subtracted, since any digit 0-9 doubled is at most 18). Adding up every digit — the doubled ones and the untouched ones — gives a total, and the check digit is whatever value 0-9 makes that total a multiple of 10.

This calculator treats whatever you type as a complete candidate number: it strips off the last digit as the "provided" check digit, recomputes what that digit should be from everything before it, and tells you whether they match. If you're instead building a new number from a known payload, type that payload followed by any placeholder last digit (0 works fine) — the "full number with correct check digit" field gives you a valid one.