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. The International Standard Book Number is the unique identifier printed on virtually every published book, and its final character is a check digit — a small piece of redundancy that lets software detect typos, swapped digits and miskeyed barcodes the moment they happen. This tool is useful for librarians, booksellers, publishers and developers building catalogue, inventory or e-commerce systems who need to catch transcription errors before storing a book identifier or charging a customer for the wrong title.

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. To convert an ISBN-10 to ISBN-13 the tool prepends the 978 Bookland prefix, drops the old check digit and recomputes the mod-10 digit; to convert an ISBN-13 back it strips 978 and recomputes the mod-11 digit. A 979-prefixed ISBN-13 has no ISBN-10 form, so the tool reports that instead of guessing.

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.