(This article has been updated. Original publication date was March 17, 2008, under the title “The Difference Between an ISBN, Bar Code, and UPC Code”. Updated October 14, 2015.)
An ISBN number stands for International Standard Book Number, and when you are assigned one for your book product, each number is totally unique. A modern ISBN has 13 digits, as of January 2007. Embedded in each number is a code that identifies your country, you as a publisher, and the unique publication. (See a future post for nerdy details.)
There is one ISBN governing body for this entire planet, and each country has an agency devoted to administering it for their country. In Canada, this is Library and Archives Canada – an office in our Federal Government, and in the USA, along with a few other countries, it is run by an organization called Bowker.
A bar code is really a computer font. When you enter a series of numbers and convert them to this font, it appears as a series of lines of varying widths which can then be read by a computer with the appropriate hardware, a bar code scanner.
An ISBN can be used to create a scannable bar code for your book, making it uniquely identifiable to booksellers who subscribe to the ISBN database. Your ISBN is a totally unique number, making it very easy to convert to a totally unique bar code using the special font.
A UPC code, or Universal Product Code, is a type of bar code, but the core number which is used to create the scannable code is derived from an entirely different source than the ISBN number. A UPC code has 12 digits rather than 13, and is grouped into two sections of 6 digits each. There is one large UPC organization that tracks and issues all the codes for the USA, called the G1-USA. They are part of the G1 organization worldwide, who also has a representative in Canada.
Why have a UPC code plus a bar code on your book? If your book is a product with wide appeal, and is likely to be sold outside of bookstores in venues like giftshops or consumer stores, a UPC code guarantees that any retailer can scan your book successfully. Not all retailers can scan an ISBN bar code, because there are more digits, and they may not have the ISBN database to relate it to.
So to sum up, both ISBN scannable codes and UPC scannable codes are types of bar codes. An ISBN is only assigned to a book or similar product, while a UPC code can be assigned to a book or virtually any product that is meant to be scanned in a retail store. My recommendation to clients is typically that they have both on the back of their books, in order to maximize market expansion options.
AND, always make sure you TEST your bar code before your book goes to press to make sure it scans properly! Your printer may offer to do this for you.