Luhn Algorithm Calculator

This Luhn algorithm calculator checks a string of digits against the Luhn checksum, the mod 10 formula used to validate credit and debit card numbers, IMEI device numbers and various other identifiers. Enter the number you want to test into the number field, digits only, spaces and other characters are ignored, and the calculator updates the result as you type. It returns three things: whether the number is valid or invalid under the Luhn test, the check digit you would append to make a new identifier valid, and the raw sum of the digits modulo 10 that the pass or fail decision is based on. Behind the scenes, the algorithm starts from the rightmost digit and doubles every second digit going left, subtracting nine from any doubled value over nine, then adds every digit together and checks whether the total is a multiple of ten. This simple method reliably catches single digit typing errors and most swaps of adjacent digits, which is why it has been used on cards and identifiers since the 1950s. Use it to sanity check a card number you have typed, to generate a valid check digit for a new identifier, or to understand how the checksum works. Passing the Luhn test only confirms the number is well formed; it does not confirm the card or identifier is genuine or active.

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