Check Your ISBN
Paste an ISBN-10 or ISBN-13 below. The tool validates the check digit, flags known placeholders, and converts between formats — entirely in your browser, no data sent anywhere.
How ISBN Validation Works
Every ISBN contains a check digit — the final digit — calculated from the preceding digits using a fixed formula. This catches single-digit typos and most transposition errors.
ISBN-13 multiplies each of the first 12 digits alternately by 1 and 3, sums the results, and derives the check digit so the total is divisible by 10.
ISBN-10 multiplies each of the first 9 digits by descending weights (10, 9, 8 ... 2), sums the results, and derives the check digit so the total is divisible by 11. Because the check digit can be 10, it is written as "X" when needed.
Why This Matters for Self-Publishers
If you are using a template, a formatter, or an older manuscript file, it may still contain a placeholder ISBN like 9781234567890. Uploading a file with a placeholder or mismatched ISBN will cause rejection by KDP, IngramSpark, or other distributors — sometimes silently, surfacing only when a reader or retailer tries to look up the book.
Catch ISBN problems before upload
Our KDP Readiness Audit checks your ISBN, metadata, and file setup in one pass.
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