An ISBN converter (from 10 to 13 digits and the reverse) is found at
http://www.isbn.org/converterpub.asp

In January 2007 the book industry moved the ISBN standard from 10 to 13 digits to accommodate the numerous books being published world-wide. 13-digit ISBNs all start with 978. The following nine digits are the same as the first nine digits in the previous 10-digit ISBN. The last digit will differ. The last digit is a check digit that comes from a mathematical equation. For those interested the equation can be found at
http://www.lac-bac.gc.ca/iso/tc46sc9/isbn/wg4n46final.pdf