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
1for the first digit,3for the second,1for 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.
Full worked example
For 978-0-306-40615-7, the validation goes like this:
| Position | Digit | Weight | Product |
|---|---|---|---|
| 1 | 9 | 1 | 9 |
| 2 | 7 | 3 | 21 |
| 3 | 8 | 1 | 8 |
| 4 | 0 | 3 | 0 |
| 5 | 3 | 1 | 3 |
| 6 | 0 | 3 | 0 |
| 7 | 6 | 1 | 6 |
| 8 | 4 | 3 | 12 |
| 9 | 0 | 1 | 0 |
| 10 | 6 | 3 | 18 |
| 11 | 1 | 1 | 1 |
| 12 | 5 | 3 | 15 |
| 13 | 7 | 1 | 7 |
Sum = 100. Since 100 is divisible by 10, the ISBN is valid.
To compute the check digit from the first 12 digits only: sum of the first 12 weighted products = 93. Then check = (10 - (93 mod 10)) mod 10 = (10 - 3) mod 10 = 7. That matches the last digit, confirming the ISBN.
Generating a check digit
If you have a 12-digit body and need the correct check digit — for example when typesetting a new barcode — enter just those 12 digits. The tool computes and displays the check digit and shows you the complete 13-digit number.
Why both 978 and 979 exist
The ISBN-13 standard originally used 978 as the only Bookland EAN prefix. As the pool of 978-based numbers was exhausted, the 979 prefix was introduced to expand capacity. Books with a 979 ISBN have no ISBN-10 equivalent, because ISBN-10 was only defined for the 978 family. The validator recognises both prefixes and flags when a prefix is neither 978 nor 979, which would indicate the number is not a book identifier at all.
Common reasons an ISBN-13 fails validation
- A single digit transcribed incorrectly (the check digit catches most single-digit errors).
- Two adjacent digits transposed (the weighted algorithm detects most transpositions).
- An ISBN-10 entered by mistake — the algorithm is different and the character count is wrong.
- A 13-digit EAN for a non-book product (for example a CD or DVD also uses EAN-13, but the prefix will not be 978 or 979).
All processing is local to your browser. Nothing you type is sent to a server.