Luhn Validation Test Case
A ready-to-run Luhn validation test case kit for QA engineers and developers verifying Mod 10 checksum logic — pass, fail, and boundary cases included.
luhn algorithm example
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The quickest Luhn algorithm example is a card number such as 79927398713. Double every second digit starting from the right, subtract 9 from any result above 9, then add all eleven digits together. If the total is a multiple of 10 the number passes the check. That is the whole method: a mod-10 checksum, not a security feature.
Key Features
- Operates on any plain digit string, typically 8 to 19 characters long.
- Reads right to left, treating the final digit as the check digit.
- Doubles every second digit and leaves the others unchanged.
- Collapses doubled values above 9 by subtracting 9.
- Catches every single-digit typo and most swaps of two adjacent digits.
- Needs no key, server call, or stored record, so it runs offline in microseconds.
- Used in payment card numbers, IMEI codes, some national IDs, and library barcodes.
How the Calculation Works
Working through 79927398713: the check digit 3 stays as is, the 1 becomes 2, the 8 becomes 16 and then 7, the 3 becomes 6, the 2 becomes 4, and the 9 becomes 18 and then 9. The digits are now 7, 9, 9, 4, 7, 6, 9, 7, 7, 2, 3, and the total is 70. Because 70 divides evenly by 10, the number is Luhn-valid. The same routine scales to any length and any issuing network, which is why it shows up wherever a cheap, dependency-free sanity check is useful.
Availability of the Method
The technique is public, unlicensed, and free to implement in any language, with no purchase or account required. Implementations are short enough to verify by hand, and any correct version will produce identical results, so you can test your own code against a known example at any time.
What does the Luhn algorithm actually prove?
It proves only that the digits are internally consistent, meaning no single digit was mistyped and no two neighbours were swapped. It says nothing about whether the number is real, active, or belongs to anyone.
Why do card numbers bother with a check digit?
A single wrong digit would otherwise reach the payment network and fail there. The check digit catches most typing errors at the point of entry, which saves time and cuts down on avoidable declines.
Can a number pass the Luhn check and still be invalid?
Yes. There are ten valid endings for any prefix, so invented numbers pass roughly one time in ten. The checksum filters typos, not fiction.