ISBN-13 Check Digit Calculator
Enter the first 12 digits of an ISBN to find the check digit, or a full number to check it, with the sum, weights and remainder shown.
How to use the ISBN-13 Check Digit Calculator
- Click the box “ISBN or the first digits of one” and type or paste your number. Hyphens and spaces are fine, so 978-0-306-40615 works as it is.
- Count the digits. 12 digits gives the ISBN-13 check digit. 13 digits checks a complete ISBN-13. 9 digits gives the ISBN-10 check digit. 10 characters checks a complete ISBN-10, and the last character may be X.
- Read the results: “Check digit”, “Complete ISBN” (without hyphens), “Is the number valid?” (for a full number), “Type of ISBN” and “Weighted sum of the digits used”.
- Look at the “Digits and weights” table. It lists each digit, its weight and the product of the two.
- Follow the steps shown below the results. They show the sum, the remainder and the check digit.
- If a full number fails, “Is the number valid?” reads something like “No: the last digit is 3 but should be 7”, and a warning follows. A mistyped or swapped digit is the usual cause, so compare your number with the source you copied it from.
- If you see an error instead, check the box. It may be empty, hold characters other than digits and a final X, have the X in the wrong place, or have a length other than 9, 10, 12 or 13.
The box starts with 978-0-306-40615, which gives check digit 7 and 9780306406157. Four presets are available: “12 digits: find digit 13”, “Check a full ISBN-13”, “A wrong check digit” and “ISBN-10 check digit”. Load one, then replace the number with your own.
What is the isbn-13 check digit?
The isbn-13 check digit is the final digit of a 13-digit ISBN. It is calculated from the first 12 digits. Because it is worked out from the other digits, it is only a test of consistency, not proof of who owns the number.
How is the ISBN-13 check digit worked out?
Multiply each of the first 12 digits by a weight that alternates 1, 3, 1, 3 and so on, from left to right. Add the 12 products together and divide the sum by 10. The check digit is 10 minus the remainder, and it is 0 when the remainder is 0.
For 978-0-306-40615 the digits and weights are 9×1, 7×3, 8×1, 0×3, 3×1, 0×3, 6×1, 4×3, 0×1, 6×3, 1×1 and 5×3. The products add up to 93. Dividing 93 by 10 leaves a remainder of 3, so the check digit is 10 − 3 = 7. The full ISBN is 9780306406157. This method follows the ISBN Users’ Manual, 7th edition, Appendix A1.1 (read 15 Oct 2026).
How do you tell if a full ISBN-13 is valid?
Take the weighted sum of the first 12 digits, then add the 13th digit. If the total divides by 10 with no remainder, the check digit is correct. For 9780306406157, the sum of the first 12 digits is 93, and 93 + 7 = 100, which divides by 10 exactly.
How is the ISBN-10 check digit different, and when is it X?
An ISBN-10 uses a different method. The 7th-edition manual does not give it, so this page follows Wikipedia’s ISBN article (read 15 Oct 2026). Multiply the first nine digits by 10, 9, 8 down to 2, and add the products. Take the remainder after dividing by 11, then subtract that remainder from 11. If the remainder is 0, the check digit is 0. If the check digit would be 10, it is written as X.
For 0-306-40615 the weighted sum is 130. The remainder after dividing by 11 is 9, so the check digit is 11 − 9 = 2. For 0-9752298-0 the weighted sum is 254, the remainder is 1, and the check digit is 10, written X.
What do the prefixes 978 and 979 mean for an ISBN-13?
A 12- or 13-digit number that does not start with 978 or 979 is not an ISBN, so the tool shows the arithmetic with a warning. Both 978 and 979 are valid prefixes for an ISBN-13. A number that starts with 979 has no ISBN-10 equivalent, so an ISBN-10 check digit cannot be produced for it.
Does a valid check digit mean an ISBN is registered?
No. The check digit only shows that the digits agree with each other under the rules above. ISBNs are issued by a national ISBN agency, so a number can pass the check and still not be assigned to your book. Confirm the assignment through the agency or the source that gave you the number.
Sources (checked 15 October 2026): isbn-international.org · en.wikipedia.org · isbn-international.org
Worked examples
| Input | Result |
|---|---|
| ISBN 978-0-306-40615 | Check digit 7, giving 9780306406157 |
| ISBN 978-0-306-40615-7 | Yes, the check digit is correct |
| ISBN 978-92-95055-12 | Check digit 4, giving 9789295055124 |
| ISBN 0-9752298-0 | Check digit X, giving 097522980X |
| ISBN 0-306-40615-3 | No: the last digit is 3 but should be 2 |
Calculated by the tool above; open it with the same values to see each step.
Frequently asked questions
Can I type hyphens and spaces into the box?
Yes. The box accepts digits with or without hyphens or spaces. The tool reads only the digits and the final X when it counts the length.
How many digits do I need to enter?
Enter 12 digits to find the ISBN-13 check digit, or 13 digits to check a full ISBN-13. For an ISBN-10, enter 9 digits to find its check digit, or 10 characters to check a full number. The number of digits decides which job the tool runs.
What does an X mean in an ISBN-10?
X stands for the value 10. It appears only as the last character of an ISBN-10 when the check digit works out to 10. The tool accepts X only in that position.
Why does the tool say “No” for my ISBN-13?
The tool says “No” when the last digit does not match the value it should have. For example, it may report that the last digit is 3 but should be 7. A mistyped or swapped digit is the usual cause, so compare the number with your source.
Does the tool upload my numbers?
No. Everything runs in your browser, and the numbers you type are not sent to a server.
Why can a 979 ISBN not have an ISBN-10?
A 979 number is a valid ISBN-13 prefix, but an ISBN-10 is only defined for numbers with the 978 prefix. The tool shows the ISBN-13 results for a 979 number and does not produce an ISBN-10.
Does a passing check mean my book is registered?
No. A passing check only confirms that the digits are internally consistent. Registration depends on the national ISBN agency that issued the number.
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