The last digit of an ISBN-13 is a check digit: multiply the first 12 digits alternately by 1 and 3, add the products, and the check digit is whatever brings the total up to the next multiple of 10. If a single digit is mistyped, the check fails, so validating the check digit catches most typing errors before an ISBN is printed on a cover or entered at a retailer. ProofSlip checks ISBN check digits in your book details on Windows and compares the title and ISBN with your EPUB, so a mistyped or mismatched number shows up before upload.
Check your ISBN, title and EPUB metadata together before you publish.
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What are the parts of an ISBN?
The International ISBN Agency’s What is an ISBN? page sets out the structure. Since 1 January 2007, ISBNs always have 13 digits, in five elements:
| Element | Length | Example in 978-92-95055-12-4 |
|---|---|---|
| Prefix | 3 digits, currently 978 or 979 | 978 |
| Registration group | 1 to 5 digits; country, region or language area | 92 |
| Registrant | Up to 7 digits; the publisher or imprint | 95055 |
| Publication | Up to 6 digits; the edition and format | 12 |
| Check digit | Always 1 digit | 4 |
Source: International ISBN Agency, checked on 17 October 2026. The ISBN Users’ Manual, 7th edition, adds that hyphens or spaces have no meaning and are only there for readability, and that when printed, the ISBN is always preceded by the letters “ISBN”.
How is the ISBN-13 check digit calculated?
The Users’ Manual, Appendix 1, gives the method:
- Weight the first 12 digits alternately by 1 and 3, starting with 1.
- Add the weighted products.
- Divide the sum by 10 and take the remainder.
- Subtract the remainder from 10. That is the check digit, with one exception: if the remainder is 0, the check digit is 0.
The manual’s worked example uses 978-92-95055-12-?:
| Digit | 9 | 7 | 8 | 9 | 2 | 9 | 5 | 0 | 5 | 5 | 1 | 2 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Weight | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 | 1 | 3 |
| Product | 9 | 21 | 8 | 27 | 2 | 27 | 5 | 0 | 5 | 15 | 1 | 6 |
The products add up to 126. 126 divided by 10 leaves a remainder of 6. 10 minus 6 is 4, so the full ISBN is 978-92-95055-12-4.
The manual also gives the compact form: check digit = mod 10 (10 minus mod 10 of the weighted sum). A quick test for a complete ISBN: the weighted sum of all 13 digits, with the check digit weighted 1, must be divisible by 10.
The manual notes that this is mainly information for system developers, and that publishers and agencies should not calculate check digits manually. In practice your ISBN agency gives you numbers that are already complete. The calculation is useful for checking, not for making up numbers.
What does the check digit catch?
The check digit is there to verify the accuracy of the number. Because of how the weights work:
- Any single mistyped digit changes the weighted sum by an amount that is not a multiple of 10, so the check fails.
- Most swapped neighbours are caught. Swapping two adjacent digits changes the sum by twice their difference, so the check only misses a swap when the two digits differ by 5, such as 1 and 6 or 3 and 8.
That is a matter of arithmetic, not policy, and it is why a check digit is a useful first test.
What does a valid check digit not prove?
A valid check digit proves only that the 13 digits are internally consistent. It does not prove:
- That the ISBN is yours. A different book’s ISBN also passes.
- That it was assigned to this format. The International ISBN Agency says each product form, such as paperback, EPUB and PDF, should have its own ISBN. KDP’s ISBN help says each print format needs its own ISBN and that an ISBN cannot be reused across formats or significantly changed new editions.
- That the details match. KDP says that if you buy your own ISBN, you must enter the same title, author and publisher that you registered, and it checks the imprint against the ISBN agency’s record. A mismatch stops publishing.
The Users’ Manual also says an ISBN can never be reused for another publication, even one assigned in error.
How do I check an ISBN on Windows?
For a quick check, this PowerShell function applies the manual’s method. Paste it into a PowerShell window, then call it with your ISBN. Hyphens and spaces are ignored.
function Test-Isbn13 {
param([string]$Isbn)
$digits = ($Isbn -replace '[\s-]', '')
if ($digits -notmatch '^\d{13}$') { return "Not 13 digits" }
$sum = 0
for ($i = 0; $i -lt 12; $i++) {
$weight = if ($i % 2 -eq 0) { 1 } else { 3 }
$sum += [int]"$($digits[$i])" * $weight
}
$check = (10 - ($sum % 10)) % 10
if ($check -eq [int]"$($digits[12])") { "Valid" } else { "Invalid: check digit should be $check" }
}
Test-Isbn13 "978-92-95055-12-4"The example returns Valid. Change the last digit to 7 and it returns “Invalid: check digit should be 4”. This is free and enough for checking one number at a time.
Check the ISBN with the EPUB and print files for the same book during the 15-day free trial.
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How does ProofSlip check book details?
ProofSlip checks print PDFs, EPUBs and Word manuscripts on your PC. Its book details check covers:
| Book details check | What it catches |
|---|---|
| ISBN check digits | A mistyped ISBN |
| Categories and keywords | Problems with the categories and keywords you plan to list |
| Title compared with your EPUB | The EPUB’s metadata still carrying a working title |
| ISBN compared with your EPUB | The paperback ISBN left in the ebook, or an old number from a previous edition |
Source: ProofSlip’s Microsoft Store listing, checked on 17 October 2026.
The comparison is the part a one-line script cannot do. An ISBN can pass its check digit and still be the wrong one for this file. Seeing the title and ISBN side by side with what is inside the EPUB catches that.
In the same run you can drop the EPUB, interior PDF and cover on the window. In the EPUB, ProofSlip checks package and navigation, broken links, missing alt text, and cover and accessibility details. On a paperback cover PDF it checks the barcode area, spine text, size, bleed and safe area. Then export every report at once.
Common ISBN mistakes before publishing
| Mistake | How it happens | How to catch it |
|---|---|---|
| A typo in one digit | Retyping from an email | Check digit validation |
| Two digits swapped | Fast typing | Check digit validation, except digits that differ by 5 |
| Paperback ISBN in the ebook | Copying front matter between formats | Compare the EPUB’s ISBN with the book details |
| Old edition’s ISBN | Reusing a previous file | Compare against your ISBN agency record |
| Details not matching the registration | Title or imprint entered differently at the retailer | Match exactly what you registered |
When is a different tool better?
- Checking one number. The PowerShell function above is free.
- Confirming ownership or registration. Only your ISBN agency’s records can do that, such as Bowker, the agency KDP names as an example, or your national agency.
- You are on a Mac. ProofSlip runs on Windows only.
Frequently asked questions
How do I calculate an ISBN-13 check digit?
Multiply the first 12 digits alternately by 1 and 3, add them, take the remainder after dividing by 10, and subtract it from 10. If the remainder is 0, the check digit is 0.
Do hyphens in an ISBN matter?
No. The ISBN Users’ Manual says hyphens or spaces have no meaning and only aid readability.
Do I need separate ISBNs for the paperback and ebook?
The International ISBN Agency says each product form should be identified separately. KDP says ISBNs are optional for ebooks on KDP, but each print format needs its own.
Does a valid check digit mean the ISBN is registered?
No. It only shows the digits are consistent. Registration is held by the ISBN agency.
Does ProofSlip upload my book details?
No. There is no account and no upload. Files never leave your PC and are never changed.
Try ProofSlip
ProofSlip is a one-time purchase in the Microsoft Store, with a 15-day free trial. Check your ISBN and book details with the EPUB and print files they belong to. For the print files, see our print-ready PDF checklist, check PDF bleed and trim box, or the ProofSlip product page.
Publish with an ISBN you have checked against the file.
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Amazon and Kindle Direct Publishing are trademarks of Amazon.com, Inc. or its affiliates. Bowker is a trademark of its owner. Microsoft, Windows and PowerShell are trademarks of the Microsoft group of companies. Other names are trademarks of their owners. ProofSlip is not affiliated with or endorsed by any of them.
Last updated 17 October 2026. Published by Automata Labs, a software company in Canada.