How is an ISBN-13 check digit calculated?

Multiply the 13 digits by alternating weights of 1 and 3 starting with 1 on the left, sum the products, and the ISBN is valid if the total is divisible by 10. The check digit is (10 minus sum-mod-10) mod 10.

What is the difference between ISBN-10 and ISBN-13?

ISBN-13 is 13 digits and uses the EAN-13 mod-10 algorithm, while ISBN-10 is 10 characters and uses a mod-11 scheme. ISBN-13 has been the standard for new books since 2007 and always starts with 978 or 979.

What do the 978 and 979 prefixes mean?

They are the GS1 prefixes that turn a book identifier into a valid EAN-13 barcode. 978 is the original Bookland prefix; 979 was added once 978 numbers ran low.

Can I generate a missing check digit?

Yes. Enter just the first 12 digits and the tool shows the correct check digit and the complete 13-digit ISBN.

Is my ISBN uploaded anywhere?

No. The calculation runs entirely in your browser, so nothing is sent to a server.

ISBN-13 Validator & Check Digit

An ISBN-13 is the 13-digit book identifier used worldwide for titles published from 2007 onward. It is structurally an EAN-13 barcode number, so it uses the same mod-10 check digit. This free validator runs that exact check and can also compute the missing check digit from the first twelve digits.

How it works

ISBN-13 uses the EAN-13 weighted modulus-10 algorithm:

Strip hyphens and spaces, leaving 13 digits.
Multiply each digit by an alternating weight, starting with 1 for the first digit, 3 for the second, 1 for the third, and so on (so odd positions get weight 1 and even positions get weight 3).
Add the products together. The ISBN is valid if the total is divisible by 10.

To generate the check digit from the first 12 digits, compute the weighted sum of those 12, then check = (10 - (sum mod 10)) mod 10.

Example

For 978-0-306-40615-7, weighting the first 12 digits as 9×1 + 7×3 + 8×1 + 0×3 + 3×1 + 0×3 + 6×1 + 4×3 + 0×1 + 6×3 + 1×1 + 5×3 = 93. Then check = (10 - (93 mod 10)) mod 10 = (10 - 3) mod 10 = 7, which matches the given digit, so the ISBN is valid.

The tool also flags whether the prefix is the expected 978 or 979. All processing is local to your browser.