Calculate the Luhn check digit for a numeric identifier prefix. The Luhn mod-10 algorithm is widely used to validate payment card numbers and other structured identifiers.
How it works
Given a digit string prefix, this converter finds the single check digit (0ā9) that makes the full number pass the Luhn validation rule. It appends candidate check digits until the completed sequence satisfies the checksum condition.
When to use it
- Create test card-like numbers for debugging and development.
- Verify that a known identifier prefix has the expected Luhn check digit.
- Generate deterministic check digits when you only have the prefix portion.
Limitations
The input must be digits only (spaces are not accepted in this subtool). Prefix length must be between 1 and 31 digits so the final identifier remains within a supported range.
Example
Input prefix 424242424242424 ā Output full number 4242424242424242 and check digit 2.