In two sentences
An ISBN is the unique identifier of one specific edition of a book: thirteen digits that encode a prefix, a language area, the publisher, the title and a check digit. It says nothing about the content or the price, but every database, buyback vendor and marketplace recognises a book by it, which makes it the key to finding out what a book is worth.
I build BiblioScan, an app that photographs a shelf and recognises every book from its spine, no barcode needed. It does not read the ISBN; it finds it, because the ISBN is how the databases name the exact edition, and therefore its value. It is also the question I get asked most: ISBN-10 or 13, why two numbers for the same book, what to do with a book that has none. Here is everything worth knowing, with a real book as the example.
What an ISBN is
ISBN stands for International Standard Book Number. Every commercially published edition of a book gets one, assigned to the publisher by the national agency (Bowker in the United States, Nielsen in the UK), and printed on the back cover under the barcode and on the copyright page. Two editions of the same text, say paperback and hardcover, have two different ISBNs; an identical reprint keeps the same one.
A little history, because it explains the two formats. In 1965 the British bookseller W. H. Smith commissioned a nine-digit numbering scheme, the SBN, from the statistician Gordon Foster. In 1970 the ISO turned it into a ten-digit international standard (ISO 2108): the ISBN-10. In January 2007, to align book numbers with the EAN barcodes used for everything else in retail and to add capacity, the ISBN moved to thirteen digits with the prefix 978. Since then a 979 prefix has been opened for countries running out of 978 ranges: 979-8 for the United States, 979-10 for France, 979-11 for Korea, 979-12 for Italy.
How to read an ISBN-13
Take a real book, Seth Klarman's Margin of Safety: ISBN 978-0-88730-510-8. The hyphens are not part of the number; they separate five blocks:
| Block | Example | What it tells you |
|---|---|---|
| Prefix | 978 | "This is a book" in the EAN system. 978 or 979. |
| Registration group | 0 | The language area or country: 0 and 1 = English-speaking (US, UK, Canada, Australia…), 2 = French, 3 = German, 84 = Spain, 88 = Italy, 4 = Japan. One to five digits long. |
| Publisher | 88730 | The publisher, here HarperBusiness. Big publishers get short codes and therefore many title numbers; a small press gets a long code. |
| Title | 510 | The number of this specific edition at this publisher. |
| Check digit | 8 | A digit computed from the other twelve, so that a typo or a misread scan can be detected. |
The publisher/title split is invisible without the hyphens (the lengths vary), but you never need it to look a book up: the thirteen digits are enough.
Verifying the check digit
This is the calculation that lets a scanner (or you) know an ISBN is valid. For an ISBN-13, multiply the first twelve digits alternately by 1 and by 3, add them up, and the check digit is whatever is needed to reach the next multiple of 10. On our example:
| Digits | 9 | 7 | 8 | 0 | 8 | 8 | 7 | 3 | 0 | 5 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Weights | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 |
| Products | 9 | 21 | 8 | 0 | 8 | 24 | 7 | 9 | 0 | 15 | 1 | 0 |
Sum: 102. The next multiple of 10 is 110, so 8 is missing: the check digit is 8, which matches the last digit. If the sum lands exactly on a multiple of 10, the check digit is 0.
An ISBN-10 uses a different rule: multiply the first nine digits by 10, 9, 8… down to 2, and the check digit completes the sum to the next multiple of 11. Since that can come out as 10, it is then written as X, which is why some older ISBNs end in a letter. The ten-digit form of our example, 0-88730-510-5, sums to 226; 5 is missing to reach 231, so the check digit is 5.
ISBN-10 vs ISBN-13: converting one into the other
Every book published before 2007 has a printed ISBN-10 and an equivalent ISBN-13 in the databases. The conversion is mechanical: drop the ISBN-10 check digit, put 978 in front of the remaining nine digits, and recompute the check digit with the 13-digit rule. 0-88730-510-5 becomes 978-0-88730-510-8. The other direction only works for ISBNs starting with 978: a book numbered under 979 (every new US self-published title with a 979-8 number, for instance) has no ISBN-10 and never will.
A detail that matters to sellers: on Amazon, a book's ASIN is its ISBN-10. A 979 book gets an arbitrary ASIN unrelated to its ISBN, which is why ISBN-10 searches fail on those titles. Our ISBN lookup accepts both formats and does the conversion itself.
What an ISBN is not
- The barcode. Since 2007 the ISBN-13 is the EAN barcode on the back: scanning the barcode means reading the ISBN. On a pre-2007 book the barcode already encodes the ISBN-13 (978 plus the ISBN-10 without its check digit) even if only the ISBN-10 is printed in clear.
- The ASIN. Amazon's in-house identifier; equal to the ISBN-10 for 978 books, arbitrary otherwise.
- The ISSN (magazines and journals, eight digits) or the ISMN (sheet music, prefix 979-0). A magazine has no ISBN.
- An identifier of the text. An ISBN identifies an edition, not a work: the same novel in paperback, hardcover, book-club edition, audiobook and ebook has five ISBNs. When you look up what a book is worth, that distinction is the whole point, because an out-of-print hardcover can be worth twenty times its paperback.
Where to look a book up by ISBN
It depends on what you want to know:
- The bibliographic record (exact title, author, publisher, year, page count): the Library of Congress catalog for US titles, WorldCat for anything held by a library anywhere, Google Books and Open Library for a quick check. Bowker's Books in Print is the publisher-side register.
- New price and availability: any retailer's search box takes an ISBN.
- Used value: a different question, and one that neither the Library of Congress nor Google Books answers. Our free ISBN lookup needs no account: type the 10 or 13 digits and you get the median price that edition actually sold for over the last twelve months, the number of sales, the competition and the buyback offers. For a fuller estimate, the book value check; for what drives the number, How much is my book worth?
What about a book with no ISBN, or no barcode?
Three cases. The book is from before the 1970s: it never had an ISBN. The book is from the 1970s to 1990s and carries a printed ISBN-10 but no barcode (barcodes only became universal in the 1980s and 1990s): type the ISBN by hand. The book is a private printing, an offprint, an association publication or a book-club edition without a number: no ISBN either.
Without an ISBN, an edition is identified by title, publisher and year, plus the printing statement on the copyright page. That is how used-book dealers and catalogs like AbeBooks work. Two practical consequences: automated buyback vendors (BooksRun, World of Books, TopDollar4Books) will not take books without a barcode, and the value of an old book is read off sales of that same edition, not off a scan. This is where the way BiblioScan works changes things: the app does not read the barcode, it reads the spine in the photo of the shelf and finds the edition in the databases, with or without a printed ISBN. A 1970s book with no barcode is recognised like any other, as long as its edition exists somewhere; for genuinely old books, a specialist dealer remains the reference.
Why the ISBN is the key to a book's value
Because it names the exact edition, the ISBN is the one identifier every source agrees on: Amazon, the buyback vendors, dealers' catalogs. When you compare prices you are comparing ISBNs, and that is where the gaps hide: in our September 3, 2026 buyback test, Margin of Safety drew $151.11 from World of Books and $86.23 from BooksRun, and had sold 43 times in the year at a median of $1,704. Without the ISBN, those three numbers are not comparable.
That is exactly what BiblioScan does: you photograph the shelf (or scan a single barcode), the AI identifies each book, and for every ISBN it finds you see the median sale price, the history, the number of sales over twelve months, the competition and the net profit, with the buyback offer next to it. The guide Where to sell old and used books: 21 sites compared then covers where to sell.
Bottom Line
ISBN = thirteen digits, one edition. Prefix, group, publisher, title, check digit. Convertible 10 ↔ 13 except under 979. It identifies; it does not value: for value you need the real sales of that edition. Look a book up by ISBN for free, or scan a whole shelf with 5 free credits.
FAQ
Where do I find the ISBN on a book?
On the back cover, above or below the barcode, and on the copyright page at the front (the reverse of the title page). It starts with 978 or 979 (thirteen digits) or has ten digits on books printed before 2007.
What is the difference between ISBN-10 and ISBN-13?
The same identifier in two formats. The ISBN-13 puts the prefix 978 in front of the nine significant digits of the ISBN-10 and recomputes the check digit. Since 2007 only ISBN-13s are assigned; books numbered under 979 have no ten-digit equivalent.
Can two books have the same ISBN?
Not by design, though publisher errors happen, mostly at small presses that reuse a number. The same book, on the other hand, has a different ISBN for every edition and format: paperback, hardcover, book club, audio, ebook.
How do I check that an ISBN is valid?
With the check digit: for thirteen digits, alternate weights of 1 and 3 and the sum must be a multiple of 10; for ten digits, weights from 10 down to 1 and the sum must be a multiple of 11 (X counts as 10). A valid ISBN is not necessarily assigned, but one that fails the calculation is definitely wrong or miscopied.
How do I find out what a book is worth from its ISBN?
By looking at what that edition actually sold for, not what it is listed at. BiblioScan's ISBN lookup gives the median of the last twelve months' sales, the number of sales and the buyback offers for any ISBN-10 or 13, free.
What do I do with a book that has no barcode?
If it has a printed ISBN-10, type it in. If it has no number at all (pre-1970s, private printing), identify it by title, publisher and year, then look at sales of that edition at used-book dealers; automated buyback sites will not take it.