ISBN-10 and ISBN-13 are both check-digit-protected identifiers for books. An ISBN-13 is really just an EAN-13 carrying a 978 or 979 prefix, while ISBN-10 uses an older, different weighting scheme in which the check character can come out as the letter X. An ASIN is Amazon's own 10-character identifier: for books it matches the ISBN-10 exactly, 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 of 10-digit ISBNs feeds into a system that expects 13-digit ones, or the reverse. This converts between them and says plainly when conversion isn't possible (see below).

It also catches one easy-to-miss data-entry mistake: an ISBN-13 with a valid checksum but a prefix that is neither 978 nor 979 is no ISBN at all, just a coincidentally valid-looking EAN-13.

Common errors

"979-prefixed ISBNs have no ISBN-10 equivalent" is a real constraint, not a bug in this tool. ISBN-10 only has room for the 978 prefix's number space, and 979 exists precisely because that space ran out, so those books never had a valid ISBN-10 in the first place.

A code that looks like a 10-character ASIN but fails ISBN-10 validation may be perfectly fine. It usually means a non-book ASIN, which never carried a checksum to validate.

Frequently asked questions