How are ISBN check digits calculated?

ISBN-10 multiplies each digit by weights 10 down to 1 and is valid when the sum is divisible by 11 (a trailing X counts as 10). ISBN-13 uses alternating weights 1 and 3 and is valid when the sum is divisible by 10.

Does a valid ISBN mean the book exists?

No. The check digit only confirms the number is internally consistent. It does not confirm that a real published title is assigned to that ISBN.

Is my input sent anywhere?

No. Validation and conversion run entirely in your browser — privacy-first, your input never leaves the device.

ISBN Validator — Gera Tools

ISBN validator

Validate the check digit of an ISBN-10 or ISBN-13, and convert between the two formats. The tool ignores hyphens and spaces and accepts a trailing X on ISBN-10s.

ISBN-10 multiplies each digit by weights 10, 9, … 1 and is valid when the sum is divisible by 11. ISBN-13 uses alternating weights 1 and 3 and is valid when the sum is divisible by 10. For a valid number, the tool also recomputes the equivalent in the other format (978-prefixed conversions).

This is privacy-first: every calculation runs locally and nothing leaves your browser. A passing check digit confirms the number is well-formed, not that a real published title exists.