A Luhn algorithm test tool takes a string of digits and returns one of two results: pass or fail. It applies the mod 10 checksum defined by Hans Peter Luhn, doubles every second digit from the right, sums the digits, and checks whether the total divides by 10. A pass means the digits are internally consistent. A fail means at least one digit is wrong or transposed. Nothing else is verified.

luhn algorithm test tool

Prerequisites

  • The digit string, with no spaces, dashes, or letters.
  • Knowledge of which end holds the check digit (the last one, on the right).
  • A calculator, spreadsheet, or the tool itself for the arithmetic.

What the Check Actually Tests

The Luhn formula is a general error-detecting checksum, not a card-specific feature. It catches any single-digit mistake and most adjacent transpositions, which is why payment networks, IMEI numbers, and national ID formats use it. ISO/IEC 7812 specifies the primary account number layout and its check digit, and the check digit is computed with the Luhn formula. The formula has no knowledge of issuing banks, account status, balances, or whether a number was ever assigned to anyone.

related article

Run the Check by Hand

  1. Write the digit string left to right.
  2. Start at the rightmost digit and skip it. That is the check digit.
  3. Double each digit in every second position as you move left.
  4. Subtract 9 from any doubled value greater than 9.
  5. Add the adjusted digits to the digits you did not double.
  6. Divide the total by 10 and look at the remainder. A remainder of 0 is a pass.

Use an Online Tool

  1. Open the tool in a browser.
  2. Paste the digit string into the input field.
  3. Read the pass or fail result.
  4. Recheck the digits if the result is a fail, since one wrong digit flips the outcome.

What a Pass Does Not Prove

A pass proves arithmetic, not authenticity. Any nine random digits plus a calculated check digit will pass. Treat the result as a format check and nothing more. PCI DSS limits how much of a primary account number a business may store or display, typically permitting only the last four digits, so a tool that keeps a full number in logs creates a compliance problem on its own. When a number fails, check for transposed digits, a missing character, or a value copied from the wrong field before assuming the source is bad.

luhn algorithm test tool

Common Mistakes

  • Including spaces or hyphens, which breaks character counting.
  • Doubling from the left instead of from the right.
  • Forgetting the subtract-9 step on doubled values above 9.
  • Treating a pass as confirmation that an account exists.