Luhn Algorithm Calculator

This Luhn algorithm calculator checks a number against the Luhn formula, the checksum used by credit card numbers, IMEI device numbers and many identifiers. Enter a number and the calculator tells you whether it passes the Luhn test, and shows the check digit you would append to a number to make it valid. The Luhn algorithm doubles every second digit from the right, subtracts nine from any result above nine, sums everything, and checks that the total is a multiple of ten. It reliably catches single digit typos and most swaps of adjacent digits, which is why it has been used since the 1950s. The method, a worked example and the assumptions are below.

Number (digits)

Luhn check digit (if building)

Sum mod 10

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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Scientific Calculator: general scientific maths.
Percentage Calculator: percentages and changes.
Prime Factorisation Calculator: factor a number into primes.

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