Checksum Test Number Generator
Generates a fictional 13-digit number whose final digit is a correctly-computed mod-10 (Luhn) checksum — for testing checksum-validation code, teaching how a check digit is derived, or exercising a validator with input that should genuinely pass. Never a real, assigned or usable number.
What this generator does
Produces a 13-digit number that passes a Luhn check, for exercising checksum and validation logic with input that a correct validator should accept.
How to use this tool
- Press generate for a checksum-valid test number.
Worked examples
- Use
- Feeding a Luhn validator a value it should accept — then changing any one digit to confirm it rejects the result
- Format
- 3479431782609 — one draw; the final 9 is the computed check digit
Common use cases
- Testing Luhn/mod-10 validation logic
- Teaching how checksum digits work
- Placeholder structured test data
How this generator works
Twelve digits are drawn at random and a thirteenth is computed by the Luhn algorithm: working from the rightmost of the twelve, every second digit is doubled and reduced by 9 if that exceeds 9, the twelve results are summed, and the check digit is whichever value brings that sum to a multiple of ten. The output passes any genuine mod-10 validator, which is what makes it useful test input rather than a value a validator should reject.
Limitations and good to know
- The Luhn digit is genuinely correct, so these pass a mod-10 check — but they are not drop-in card or account numbers, because the leading digits are not a real issuer prefix and nothing here is assigned to anyone.
Illustrative example data only — never a real, usable credential, address or in-use identifier.
Related generators
- ISBN Example GeneratorAn ISBN-13-shaped example number with a correct check digit, for test data that has to pass validation.
- Hamming Code GeneratorEncodes four bits as seven, flips one on purpose, and shows the syndrome pointing straight at the corrupted position.
- Barcode Number Example GeneratorA 12-digit UPC-A-shaped example number with a correct check digit — not a real, assigned barcode.
- Run-Length Encoding GeneratorEncodes text as runs of repeated characters, decodes it back to prove it is exact, and says plainly when the result is longer.