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.

Why does an ISBN-10 sometimes end in X?

ISBN-10 uses a modulo-11 check, so the check value can be 0 to 10. Because 10 needs a single character, it is written as the letter X. This only happens in the final position of an ISBN-10, never in an ISBN-13.

How do I convert ISBN-10 to ISBN-13?

Drop the ISBN-10 check digit, prefix 978, then recompute the ISBN-13 check digit with the alternating 1-3 weights. The tool does this automatically and shows the equivalent number for any valid input.

What is the 978 or 979 prefix?

ISBN-13 numbers begin with a GS1 prefix — 978 (the original "Bookland" range) or the newer 979. ISBN-10s map only to 978-prefixed ISBN-13s; 979 numbers have no ISBN-10 equivalent.

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. This is useful for librarians, booksellers, publishers and developers building catalogue systems who need to catch transcription errors before storing a book identifier.

How it works

The tool ignores hyphens and spaces, detects the length, and verifies the check digit:

ISBN-10: multiply the digits by weights 10, 9, 8 … 1 (a trailing X counts as 10) and sum. Valid when the total is divisible by 11.
ISBN-13: multiply the digits by alternating weights 1 and 3 and sum. Valid when the total is divisible by 10.

For a valid number it also recomputes the equivalent in the other format (978-prefixed conversions).

Example

ISBN-10 0-306-40615-2: 0×10 + 3×9 + 0×8 + 6×7 + 4×6 + 0×5 + 6×4 + 1×3 + 5×2 + 2×1 = 0 + 27 + 0 + 42 + 24 + 0 + 24 + 3 + 10 + 2 = 132. 132 ÷ 11 = 12 exactly, so the check digit is valid. Its ISBN-13 equivalent is 978-0-306-40615-7.

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.