Validate ISBN-10 and ISBN-13 numbers, verify their checksum, convert between formats, and identify the registration group prefix
The International Standard Book Number uniquely identifies books. ISBN-10 (pre-2007) uses a mod-11 checksum where each of the first 9 digits is multiplied by its position (1–9) and the sum must be divisible by 11; the check digit 10 is written as X. ISBN-13 is a 13-digit EAN-13 with a mod-10 checksum (digits alternately multiplied by 1 and 3). All ISBN-10 numbers can be converted to ISBN-13 by prepending 978 and recomputing the check digit. ISBN-13 numbers starting with 979 have no ISBN-10 equivalent.