ISBN-10 Validator

ISBN-10: the legacy book number with a mod-11 check digit that can be X

The ISBN-10 is the original International Standard Book Number format: 10 digits defined by ISO 2108, used on every book published from 1970 until the 13-digit switchover on 1 January 2007. Millions of older books still carry only an ISBN-10, so validating and converting it remains essential for catalogues and second-hand inventories. This tool checks the structure and the check digit.

Its checksum is a weighted mod-11: multiply the digits by 10, 9, 8, …, 2 for the first nine, add the check digit times 1, and the total must be divisible by 11. Because the check digit must represent the value 10 in some cases, ISBN-10 uniquely allows a final X (Roman ten).

The four parts of 0-306-40615-2

• Registration group (0): language/region (0/1 English, 85 Brazil, 972 Portugal).
• Registrant (306): the publisher.
• Publication (40615): the title/edition.
• Check digit (2): the mod-11 digit, possibly X.

ISBN-10 ⇄ ISBN-13

• 10 → 13: drop the ISBN-10 check digit, prepend 978, recompute the EAN mod-10 check digit.
• 13 → 10: only possible for 978 numbers — strip the prefix and the mod-10 digit, recompute the mod-11 digit.
• 979 books: have no ISBN-10 equivalent at all.

Common pitfalls

• The X is uppercase only: a lowercase x should be normalised; X equals the value 10 and is only ever the last character.
• Different algorithm from ISBN-13: mod-11 (weights 10…1) vs mod-10 (weights 1,3). Don't reuse the same checksum function.
• Hyphen positions vary: strip hyphens before validating.
• Valid checksum ≠ real book: the algorithm only proves the number is well-formed.

FAQ

Why does ISBN-10 use X? The mod-11 check value ranges 0–10; the digit 10 is written as the Roman numeral X so it still fits one character.

Are new books still issued ISBN-10? No. Since 2007 only ISBN-13 is assigned, but old stock and reprints keep their ISBN-10.

Is the check digit ever X in ISBN-13? No — only ISBN-10 can end in X; ISBN-13 always ends in 0–9.