How the Luhn checksum works
Starting at the right, every second digit is doubled. Values over nine have nine subtracted. A complete identifier passes when the sum is divisible by ten. Generation chooses the final digit that makes that true.
What should I enter?
Enter ASCII digits, optionally separated by spaces or hyphens. Leading zeros are preserved. Generate mode accepts a payload without its check digit; validate mode expects the full identifier with at least two digits. Up to 1,000 digits are supported.
What does a valid checksum prove?
A passing checksum only checks arithmetic. It does not prove that an identifier was issued, belongs to anyone, or is active. Luhn catches many typing errors but not every transposition.
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