Collecting guide
ISBNs Explained: ISBN-10 vs ISBN-13 and Why Old Books Lack One
What an ISBN is, how ISBN-10 and ISBN-13 differ, how the check digit works, where to find the ISBN on a book, and why books before about 1970 don't have one.
The International Standard Book Number (ISBN) is the identifier that lets bookstores, libraries and marketplaces tell one edition of a book from another. Understanding it makes buying and selling used books much easier.
What an ISBN identifies
An ISBN identifies a specific edition and format of a book from a specific publisher. The hardcover, paperback, large-print and ebook versions of the same title each have different ISBNs, and so do later editions with changed content. That’s why, when edition matters, searching by ISBN is the most precise way to find a book.
A short history
The ISBN system grew out of a nine-digit Standard Book Numbering scheme used in the UK in the 1960s, and was adopted internationally around 1970 as a ten-digit number. From January 2007, all new ISBNs became 13 digits, aligning with the barcodes used on retail products. As a result:
- Books from before about 1970 usually have no ISBN.
- Books from about 1970 to 2006 typically show an ISBN-10, and many later printings show both.
- Books from 2007 onward have an ISBN-13.
ISBN-10 vs ISBN-13
Every ISBN-10 has an equivalent ISBN-13: add the prefix 978 to the first nine digits and recalculate the final check digit. Newer ISBNs may begin with 979, which has no ISBN-10 equivalent. Our ISBN Converter does the conversion instantly in both directions.
How the check digit works
The last digit of an ISBN is a check digit that catches typos:
- ISBN-10: multiply the first nine digits by 10, 9, 8 and so on down to 2, add them up, and choose the final digit so that the total (including the check digit multiplied by 1) divides evenly by 11. If that digit would be 10, it’s written as X.
- ISBN-13: multiply the digits alternately by 1 and 3, add them up, and choose the final digit so the total divides evenly by 10.
If a buyback site or catalogue says your ISBN is invalid, a typo or swapped pair of digits is the usual cause. The converter shows what the check digit should be, so you can spot the mistake.
Where to find the ISBN
- Back cover: above or below the barcode, usually labelled “ISBN”
- Copyright page: the reverse of the title page
- Dust jacket flap or back panel on hardcovers
- Spine on some academic books
The barcode itself (EAN-13) encodes the ISBN-13. A second, smaller barcode to the right sometimes encodes the price.
ISBNs and used books
- Buying: search by ISBN when you need an exact edition, such as a textbook or a specific collectible printing.
- Selling: buyback services quote by ISBN; enter it exactly.
- Collectibles: an ISBN doesn’t indicate printing. A first printing and a tenth printing usually share the same ISBN, so check the number line too.
- Books without ISBNs: search by title, author, publisher and year; our compare tool handles both.
Frequently asked questions
Where is the ISBN on a book?
On most books it is printed above the barcode on the back cover and on the copyright page. On older books, if present, it may be on the copyright page or the jacket.
Do two editions of the same book share an ISBN?
No. Each edition and format - hardcover, paperback, ebook, audiobook - gets its own ISBN. That's why ISBN searches find one exact edition.
Why does my old book have no ISBN?
The ISBN system began around 1970. Books published before that generally have no ISBN, although some later reprints of older books do.