ISBN lookup: find a book, check digit, ISBN-10 to 13

Enter an ISBN: the tool validates the check digit, converts between ISBN-10 and ISBN-13 and shows title, author, publisher and cover from Open Library.

ISBN check digit and 10-to-13 conversion: how it works

The last character of an ISBN is a check digit: for an ISBN-10, the digits multiplied by 10, 9, 8… down to 1 must add up to a multiple of 11 (X stands for 10); for an ISBN-13, the digits weighted alternately by 1 and 3 must add up to a multiple of 10. To go from 10 to 13 digits, put 978 in front of the first 9 digits and recalculate the check digit.

What the tool does

The check digit is verified in your browser. If it is wrong, the tool shows the expected character, which catches a typo without any request. A valid ISBN is converted to the other format and sent to the Open Library Books API, the Internet Archive's collaborative catalogue. The record shows the title, authors, publisher, publication date as entered in the catalogue, page count, subjects and the cover when there is one.

Worked example

Le Petit Prince published by Gallimard has the ISBN-10 2-07-061275-9. Weighted sum of the first 9 digits: 2×10 + 0×9 + 7×8 + 0×7 + 6×6 + 1×5 + 2×4 + 7×3 + 5×2 = 156. 156 divided by 11 leaves 2, so the check digit is 11 − 2 = 9. As an ISBN-13: 978207061275, weighted 1-3-1-3… sum = 112, check digit 10 − 2 = 8, giving 978-2-07-061275-8.

Limits

  • Open Library does not know every book. A missing book does not mean a wrong ISBN.
  • ISBNs starting with 979 have no 10-digit equivalent.
  • Records are entered by libraries and volunteers, so dates and subjects may be incomplete.

Sources and dates: Open Library, Books API documentation; Open Library, data licensing; ISO 2108 standard (International Standard Book Number) (checked 15/10/2026)

Frequently Asked Questions

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

Drop the ISBN-10 check digit, put 978 in front of the remaining 9 digits and recalculate: add the 12 digits weighted 1, 3, 1, 3…, and the check digit is 10 minus the remainder of that sum divided by 10 (0 if the remainder is 0). Example: 0-14-032872-1 becomes 978-0-14-032872-1.

What does the X at the end of an ISBN mean?

In an ISBN-10 the check digit is computed modulo 11, so it can be 10; it is written X to keep a single character. An ISBN-13 is computed modulo 10 and never contains an X.

Where do I find a book's ISBN?

It is printed on the back of the title page (the copyright page) and under the barcode on the back cover; the 13 digits of a book's EAN barcode are its ISBN-13. Since 1 January 2007, new ISBNs have been issued in the 13-digit format.

Why is my book not found when the ISBN is valid?

A valid check digit only proves that the number is well formed; Open Library does not list every edition, especially recent or self-published books. Try the other ISBN format, another edition, or the Library of Congress or British Library catalogue.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.