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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Number (digits)

Valid (passes Luhn)

Luhn check digit (if building)8

Sum mod 100

The formula

Starting from the rightmost digit, double every second digit; if a doubled value exceeds 9, subtract 9. Sum all the digits. The number is valid if this total is a multiple of 10. The check digit makes the total reach the next multiple of ten.

Worked example

The number 79927398713 passes the Luhn test: applying the doubling and summing gives a total that is a multiple of 10, so it is valid. Enter it to confirm.

Frequently asked questions

What is the Luhn algorithm?

A simple checksum that doubles alternate digits and checks the total is a multiple of ten. Credit cards and IMEIs use it.

Does passing Luhn mean a card is real?

No. It only catches typing errors. A number can pass Luhn yet not be a genuine, active card.

What errors does it catch?

All single digit mistakes and most swaps of two adjacent digits.

Who this calculator is for

This calculator is for developers, students and anyone validating card or identifier numbers.

What this calculator assumes

You enter a string of digits.
The standard Luhn (mod 10) algorithm is used.
Spaces and other non-digits are ignored.

Formula and sources

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