Publishing glossary
Free ISBN validator
Validate any ISBN and get the clean ISBN-13 form.
Paste an ISBN-10 or ISBN-13. The validator checks the check digit, converts to ISBN-13, and formats it for your metadata.
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A worked ISBN-13 check, digit by digit
The check digit is arithmetic you can reproduce by hand. Take 978-0-306-40615-7, drop the last digit, and multiply the first twelve digits alternately by 1 and 3: 9x1 + 7x3 + 8x1 + 0x3 + 3x1 + 0x3 + 6x1 + 4x3 + 0x1 + 6x3 + 1x1 + 5x3 = 93. The check digit is whatever brings that total up to a multiple of ten, so (10 - (93 mod 10)) mod 10 = 7, and the number ends in 7 exactly as published.
ISBN-10 uses the same idea with weights running from 10 down to 2 and modulo 11, which is why its last character can be X when the remainder is 10. The validator accepts both forms, converts an ISBN-10 into the 978-prefixed ISBN-13 by recomputing the check digit, and reports a failure as either check-digit or length so you know which fix to make.
| You paste | The validator says | Why |
|---|---|---|
| 978-0-306-40615-7 | Valid, 978-0-306-40615-7 | Weighted sum is 93, so the check digit is 7 |
| 0-306-40615-2 | Valid, 978-0-306-40615-7 | ISBN-10 passes modulo 11 and converts to the 978 prefix |
| 0-306-40615-3 | Invalid: check-digit | Last character is not the modulo 11 result |
| 978-0-306-40615-8 | Invalid: check-digit | The weighted sum needs 7, not 8 |
| 978030640615 | Invalid: length | Twelve digits is a shortened string, not an ISBN |
What the check-digit math actually proves
An ISBN is not a random serial: the last digit is calculated from the first twelve. A transposed digit nearly always fails the check. Amazon, Ingram, and every retailer key the edition record by the whole number, so a bad ISBN either fails the lookup or points the store to someone else's book.
The validator converts ISBN-10 to the ISBN-13 form you need in KDP metadata, and it formats the number without stray spaces or hyphens so you can paste it straight into your listing.
- Retailer lookups use the literal string — keep it exact
- 978 and 979 prefixes are both valid; the check covers both
- An IBNS-style typo in your metadata is invisible until the store cannot find the edition
Before you paste it into the copyright page
Decide first whether you use Amazon's free KDP ISBN (perfectly fine in the KDP store) or a purchased ISBN that allows broad distribution elsewhere. The number itself does not change, but where it appears does: print editions need the ISBN in the copyright page; the ebook's ISBN is metadata only.
If a number already belongs to an existing edition — for example, a second edition of a book you published — do not reuse it. New content, changed trim size, or a re-issued format each need their own number. Keep one row in the metadata checklist for every ISBN you own.
FAQ
Frequently asked questions
What does the validator do if the number is invalid?
It marks the failed check digit and shows both the offending digit and the corrected ISBN-13, so you can fix the source instead of guessing which digit slipped.
How is an ISBN check digit calculated?
For ISBN-13, multiply the first twelve digits alternately by 1 and 3, add them, then take (10 - total mod 10) mod 10. For 978-0-306-40615-7 the weighted sum is 93, so the check digit is 7. ISBN-10 weights run from 10 down to 2 modulo 11, which is why its last character can be X.
Can I convert an ISBN-10 to ISBN-13?
Yes. Prefix the first nine digits with 978 and recompute the check digit: 0-306-40615-2 and 978-0-306-40615-7 are the same edition. An ISBN-13 beginning with 979 has no ISBN-10 equivalent, because the ISBN-10 form only exists in 978 space.
Why does it say length instead of check-digit?
Length means that after removing spaces and hyphens the string is neither 10 nor 13 characters, or that a 13-digit input does not start with 97. Check-digit means the length is right but the last character is not what the arithmetic produces, so a digit somewhere in the middle is wrong.
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