How it works
EPUBCheck is the W3C's official validator for EPUB files. Stores and distributors run it on what you upload, and a file with errors is usually turned away.
The checker runs EPUBCheck on your file and lists its errors and warnings, the first 100 of them, with the file and line each points to. A dozen common messages, such as a link to a file that isn't in the book, also get a plain-English explanation.
Your file is deleted straight after the check: nothing is kept.
A worked example
A typical result reads "2 errors and 1 warning": for example, a chapter linking to a page that isn't in the book, a picture listed in the wrong format, and a missing description. Fix the errors in the program that made the EPUB, export again, and check again.
Questions authors ask
- Is this the real EPUBCheck?
- Yes: the W3C's own EPUBCheck, the same tool the stores use.
- Do warnings matter?
- Errors stop most stores; warnings usually don't, but they often point to something readers will notice, such as a missing table of contents entry.
- Does KDP need a valid EPUB?
- KDP converts what you upload, so it is more forgiving, but an EPUB that passes EPUBCheck is the safe way to make every store accept the same file.
Sources
Checked on 17 October 2026 against W3C: EPUBCheck. Stores change their rules: their own pages have the final word.
Build the whole edition here. Ebook, paperback and hardcover from one manuscript, covers from your own artwork, ISBNs and a paste-ready kit for every store. You upload and publish yourself.
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