ISBN Validator (ISBN-10 / ISBN-13)

An ISBN identifies one edition of one book — a hardback, its paperback and its ebook each get their own. The tool detects the format from the length, checks the digit, and converts between the two where a conversion exists. ISBN-10 uses a modulo-11 checksum over weights ten down to one, which is why the last character can be an X: eleven possible remainders need eleven symbols, and there is no single digit for ten. ISBN-13 uses the modulo-10 scheme shared with EAN-13 and UPC, alternating weights of one and three, so an ISBN-13 is also a scannable retail barcode.

Conversion is asymmetric and that catches people out. An ISBN-10 always converts to an ISBN-13 by prefixing 978 and recomputing the check digit. Going back only works when the ISBN-13 starts with 978, because that prefix was allocated to mirror the old ten-digit space exactly. The 979 prefix, brought in when 978 began to fill up, has no ten-digit counterpart at all — 979-11 titles, which include a large share of recent Korean publishing, simply have no ISBN-10 and never will. A system that still demands ten digits is a system that cannot represent books published in the last few years.

The two checksums are not equally strong, and the older one is better. Modulo 11 catches every single-digit error and every transposition of two digits. Modulo 10 catches every single-digit error but misses some transpositions — specifically, swapping two adjacent digits that differ by five leaves the total unchanged, because the weights differ by two and twice five is ten. The valid ISBN-13 9780306406157 becomes 9780306401657 when its 6 and 1 change places, and that string passes the check as cleanly as the original. So a checksum pass is evidence, not proof.

Neither check says a book exists. The digits are internally consistent; whether that edition was ever published, and whether the title you have matches it, needs a catalogue lookup. Validation runs entirely in your browser, and hyphens and spaces are ignored, so an ISBN copied from a copyright page in any of its usual groupings works as pasted.