ISBN validator
An ISBN carries a check digit, but not the same one depending on its length: modulo 11 for the old 10-character numbers — hence the X you sometimes see — and the GS1 check digit for ISBN-13, which are proper barcodes.
Dashes and spaces are ignored. A trailing X is accepted on ISBN-10.
How the check works
- 1
Length picks the algorithm
Ten characters: modulo 11, weights from 10 down to 1, the check character possibly X to represent 10. Thirteen digits: the GS1 check digit, identical to an EAN-13 barcode's.
- 2
The check digit is recomputed and compared
The expected character appears next to yours: a failing ISBN almost always comes from a transposed digit during copying.
- 3
The conversion is offered
A valid ISBN-10 is converted to ISBN-13: prefix 978 is added, the old check digit dropped, the new one computed.
Why the trailing X
An ISBN-10 check is a modulo 11: the remainder can be 10, a value no single digit represents. The standard settled on the letter X, which therefore means 10 and nothing else. An X anywhere but the last position is not an ISBN.
ISBN-13 does not have this problem: its check is a modulo 10 and always fits in one digit. That is also why converting 10 → 13 changes the check digit.
What an ISBN actually identifies
An edition, not a work. The hardback, the trade paperback, the ebook and the mass-market edition of the same novel carry four distinct ISBNs. A catalogue de-duplicating on ISBN therefore keeps all four — by design.
Frequently asked questions
What is the difference between ISBN-10 and ISBN-13?
ISBN-10 is the old format, used until 2007. ISBN-13 adds the 978 prefix (or 979 for recent allocations) and recomputes the check digit under the GS1 rule, which makes it a valid EAN-13 barcode and lets a book be scanned at a till like any other product.
Can an ISBN-13 be converted back to ISBN-10?
Only if it starts with 978: drop the prefix and recompute the check digit modulo 11. A 979 ISBN-13 has no 10-digit equivalent, since that range was opened precisely when the 978 space ran out.
What does the digit after 978 mean?
It is the registration group: a language or geographic area. 0 and 1 for English, 2 for French, 3 for German, 4 for Japanese, 88 for Italian, 84 for Spanish. It does not say where the book was printed, only which agency the publisher belongs to.
Does a valid ISBN correspond to an existing book?
No. The check digit only proves the digits are consistent. To find out whether an ISBN has been assigned and to which title, you need a catalogue — the national ISBN agency, the Library of Congress, or a commercial bookseller database.
The same check over API
This tool runs in your browser. To run the same check from your code, in bulk or server-side, the API answers in JSON.
Read the documentation