What this calculator does
Luhn algorithm (checksum) works out validates a number sequence or computes the check digit that makes it valid. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaFrom the rightmost digit, double every second digit (subtract 9 if over 9); valid when the total digit sum is divisible by 10
The inputs explained
| Field | What to enter |
|---|---|
| Digits (spaces allowed) | A list of numbers, separated by commas or spaces. |
| Mode | Choose from Check if valid, Generate a check digit for these digits. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with mode
Every other input is held at the calculator’s starting values. Select any row to load that scenario.
How the answer changes with mode
| Mode | Valid by the Luhn check? | Digit sum (mod 10) | Digits checked |
|---|---|---|---|
| Check if valid | Yes | 0 | 16 |
| Generate a check digit for these digits | 8 | 45391488034364678 | 16 |