ISBN-10 and ISBN-13 are both check-digit-protected identifiers for books - ISBN-13 is really just an EAN-13 with a 978 or 979 prefix, while ISBN-10 uses an older, different weighting scheme where the check character can be the letter X. An ASIN is Amazon's own 10-character identifier: for books it's identical to the ISBN-10, but for everything else it's an opaque code Amazon assigns internally with no public checksum at all.

When you'd use this

Publishing and bookselling workflows often need both formats - an older catalog with 10-digit ISBNs feeding into a system that expects 13-digit ones, or vice versa. This converts between them and tells you immediately when that's not possible (see below).

It's also useful for catching a specific, easy-to-miss data-entry mistake: an ISBN-13 with a valid checksum but a prefix that isn't 978 or 979 isn't an ISBN at all, just a coincidentally valid-looking EAN-13.

Common errors

"979-prefixed ISBNs have no ISBN-10 equivalent" isn't a bug in this tool - it's a real constraint. ISBN-10 only has room for the 978 prefix's number space; 979 was introduced specifically because that space ran out, so there was never a valid ISBN-10 for those books.

A code that looks like a 10-character ASIN but fails ISBN-10 validation isn't necessarily wrong - it just means it's a non-book ASIN, which never had a checksum to validate in the first place.

Frequently asked questions