ISBN-13 Check Digit Calculator

ISBN-13, the 13-digit edition of the International Standard Book Number that replaced the older 10-digit format as the global standard in 2007, always starts with the "Bookland" prefix 978 or 979 and ends with one check digit computed from the other 12. That digit comes from a weighted sum: multiply the 12 prefix digits alternately by 1 and 3 (starting with 1 for the leftmost digit), add up the 12 products, and the check digit is whatever value 0-9 makes that total a multiple of 10 — formally, (10 − (sum mod 10)) mod 10.

This is the exact same checksum used by EAN-13 retail barcodes generally; an ISBN-13 is mechanically just an EAN-13 number reserved for books under the 978/979 prefix range, which is why a printed book's ISBN-13 doubles as its scannable barcode.