ISBN 10 to 13 converter and check digit calculator

Why a book has two ISBNs

ISBNs were ten digits long until the end of December 2006, and since 1 January 2007 they have always had 13, according to the International ISBN Agency. Books published before then carry the old ten-digit number; many from the changeover years print both, which Library and Archives Canada says publishers could do for books issued during that time. The 13-digit form is the same number as the book’s EAN-13 barcode, which is why shops and libraries now use it everywhere. A book can also have several ISBNs for another reason: the agency’s Users’ Manual says a separate ISBN goes to each separate edition or format, so the hardback, paperback, PDF and EPUB of one book each get their own.

How the check digit works

The last digit is a check on all the others, so most typing mistakes give an invalid number. For an ISBN-13, the Users’ Manual (Appendix 1) multiplies the first twelve digits alternately by 1 and 3, adds the products and divides by 10. The check digit is 10 minus the remainder, or 0 when the remainder is 0. Its own example, 978-92-95055-12-?, adds up to 126; 126 ÷ 10 leaves 6, and 10 − 6 = 4. The result card shows the same working for any ISBN you type.

The old ten-digit check uses a different rule. The agency’s current manual no longer prints it, so this part comes from Wikipedia’s ISBN article, which cites the 2001 edition of the manual: the ten digits, weighted 10, 9, 8 and so on down to 1, must add up to a multiple of 11. When the check value needed is 10, it is written as X. That is why some ten-digit ISBNs end in X and no 13-digit ISBN ever does.

Converting 10 to 13, and why 979 can’t go back

Converting from 10 to 13 digits keeps the middle nine digits, adds 978 in front and recalculates the check digit with the 13-digit rule, so the last digit usually changes. Converting back drops the 978 and recalculates with the ten-digit rule. That only works because every ten-digit ISBN was, in effect, a 978 number. The manual says GS1 has made both 978 and 979 available as prefixes. A ten-digit ISBN has no room for a prefix, so dropping 979 would produce a number that belongs to the 978 range. ISBNs that start 979 therefore have no ten-digit form; the agency’s range data currently lists 979 groups for France, Korea, Italy, Spain and the United States.

Where the hyphens go

The group, registrant and publication parts vary in length: a large publisher gets a short registrant number and room for many titles. So you can’t hyphenate an ISBN by counting digits. The tool uses the International ISBN Agency’s published range data (its RangeMessage file), bundled into this page as packaged in version 2.0.13 of the open-source isbn3 library, released on 1 November 2026. It never goes online to look anything up. If a number falls in a range the agency hasn’t allocated yet, the tool says so rather than guessing. The manual is clear that hyphens or spaces have “no lexical significance”: 9789295055124 and 978-92-95055-12-4 are the same ISBN.

The barcode on the back cover

The barcode on a book is an EAN-13 symbol that encodes the ISBN-13’s thirteen digits. The manual says the ISBN must be printed in human-readable form, preceded by “ISBN”, immediately above the barcode, and recommends the lower right of the back cover near the spine. A smaller block of bars to the right, where there is one, is a five-digit add-on: the manual says it shows the price in the United States and Canada, and elsewhere may only carry a publisher’s in-house code. It isn’t part of the ISBN.

Common mistakes

• Keeping the old check digit. 0-306-40615-2 becomes 978-0-306-40615-7, not …-2. Always recalculate.
• Reading X as an error. X is a valid last character of a ten-digit ISBN.
• Typing the letter O or I for zero or one. The tool names any character it can’t accept.
• Reading the group as the printing country. The group is a country, region or language area: 978-0 and 978-1 are both the English language group.
• Trusting a “fixed” check digit. When the last digit is wrong, another digit was usually mistyped. Check the book rather than taking the suggested digit.

For batch work, paste a column copied from a spreadsheet, or a CSV whose header has a column named ISBN, and download the results as CSV. To keep track of the books themselves, add them to the home library catalogue, which checks each ISBN the same way.