Luhn Algorithm Validator

How the Luhn algorithm works

1. Starting from the rightmost digit, double every second digit.
2. If doubling produces a number greater than 9, subtract 9 (equivalent to adding the two digits).
3. Sum all digits (doubled and unchanged).
4. If the total is divisible by 10, the number is valid.

Example: 79927398713 passes the check — the processed digits sum to 70. Change any single digit and the sum will no longer be divisible by 10.

Implementation

JavaScript

function isValidLuhn(number) {
  const digits = number.replace(/\D/g, '');
  let sum = 0;
  let double = false;

  for (let i = digits.length - 1; i >= 0; i--) {
    let digit = Number(digits[i]);

    if (double) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }

    sum += digit;
    double = !double;
  }

  return sum % 10 === 0;
}

Python

def is_valid_luhn(number: str) -> bool:
    digits = [int(c) for c in number if c.isdigit()]
    total = 0

    for index, digit in enumerate(reversed(digits)):
        if index % 2 == 1:
            digit *= 2
            if digit > 9:
                digit -= 9
        total += digit

    return total % 10 == 0

Luhn is not the whole story

A number can pass Luhn and still be invalid: the BIN may not exist, the length may be wrong for the network, or the card may simply not be issued. Luhn only detects accidental errors such as typos. For a full check, combine it with the BIN checker, and for practice numbers use the test card generator.