The International Standard Book Number (ISBN) is used to uniquely identify books.

Each book¹ has its ISBN printed on it somewhere, usually the back cover and the copyright page. This number is ten digits long; the first nine are the actual number, and the last one is a check digit. This is used to ensure that the ISBN is entered correctly when typed in by hand.

The Parts of the ISBN

The nine digits are split into three parts. When written, these are sometimes separated by hyphens. These parts are not of fixed length.

Part one is the country code. This is the book's language or country of origin. Some of the more common ones are:

• 0,1 = English

• 2 = French

• 3 = German

• 4 = Japanese

Part two is the publisher number. A small publisher will have a long number, and a large publisher will have a short number.

Part three is the book number. Note that the small publisher has fewer digits available for books. If a publisher runs out of numbers, then it is assigned a new publisher number.

The Check Digit

The check digit is easy to calculate.

Take the first nine digits of the ISBN. Multiply the first one by one, the second by two, the third by three and so on, up until the ninth digit by nine. Add up all these results and divide by 11. The remainder is the check digit.

For example, if the ISBN is 1852864117:

1 x 1 = 1
8 x 2 = 16
5 x 3 = 15
2 x 4 = 8
8 x 5 = 40
6 x 6 = 36
4 x 7 = 28
1 x 8 = 8
1 x 9 = 9

1 + 16 + 15 + 8 + 40 + 36 + 28 + 8 + 9 = 161

161 / 11 = 14 remainder 7

Therefore the check digit is 7, which is correct.

This is useful because if any of the digits are transposed when typing - eg 46 instead of 64 - then the check digit will be wrong, and the error is immediately apparent.