Gera Tools

ISBN-10 Validator & Check Digit

Validate ISBN-10 and compute missing check digits

Validate any ISBN-10 with the ISO 2108 mod-11 checksum, or enter the first 9 digits to compute the correct check digit (including the X case). Hyphens are ignored and everything runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

How is an ISBN-10 check digit calculated?

Multiply the first 9 digits by weights 10 down to 2, add the check digit times 1, and the ISBN is valid if the total is divisible by 11. The check digit is the value 0–10 that makes the sum divisible by 11, where 10 is written as X.

An ISBN-10 (International Standard Book Number, 10 characters) is the older book identifier defined by ISO 2108 and used for titles published before 2007. The final character is a check digit that lets you catch typos. This free validator runs the official mod-11 checksum and can also compute the correct check digit when you enter only the first nine digits.

How it works

ISBN-10 uses a weighted modulus-11 scheme:

  1. Strip hyphens and spaces, leaving 10 characters.
  2. Multiply the digits by descending weights: position 1 by 10, position 2 by 9, down to position 9 by 2, and the check character by 1.
  3. The check character may be 0–9 or X, where X represents the value 10.
  4. Add all the products together. The ISBN is valid if the total is divisible by 11.

To generate the check digit for the first nine digits, compute (11 - (weighted sum) mod 11) mod 11; a result of 10 is written as X.

Step-by-step example

Validate 0-306-40615-2:

PositionCharacterWeightProduct
10100
23927
3080
46742
54624
6050
76424
8133
95210
10212

Sum = 132. 132 ÷ 11 = 12 exactly, so the ISBN is valid.

Now try 0-306-40615-3. The check digit 3 contributes 3×1 = 3, making the total 133. 133 mod 11 = 1, which is not 0, so this ISBN fails. The tool shows the expected check digit (2) so the typo is immediately obvious.

Computing a missing check digit

Enter just the first 9 digits — for example 030640615 — and the tool calculates:

weighted sum = 130 (same as above, without the check digit position)
check = (11 - 130 mod 11) mod 11
      = (11 - 9) mod 11
      = 2

So the correct check digit is 2, giving the full ISBN 0-306-40615-2.

If the formula yields 10, the check character is written as X — this is the only position where X can appear in an ISBN-10, and it is not an error.

Why ISBN-10 was replaced

ISBN-10 was the global book identifier until 2007, when the publishing industry switched to ISBN-13 (a 13-digit EAN-13 barcode format). The reason was practical: ISBN-10 had only 10 digits and the supply of available numbers was being exhausted. ISBN-13 prepends 978 (or the newer 979) as a GS1 prefix, using a different mod-10 check. Every ISBN-10 maps to exactly one ISBN-13 by prepending 978; the reverse is only true for 978-prefixed ISBN-13s.

Where ISBN-10 is still encountered

Although publishers no longer assign new ISBN-10s, they remain relevant for:

  • Library catalogues that were built before 2007 and have not fully migrated.
  • Second-hand bookselling platforms where older inventory still carries 10-digit barcodes.
  • API integrations with book databases that return both formats for backwards compatibility.
  • Data quality checks on legacy records to detect transcription errors before migrating to ISBN-13.

This validator handles all these cases: it validates ISBN-10 with the mod-11 check and can also convert a valid ISBN-10 to its ISBN-13 equivalent. All processing is local — nothing leaves your browser.