Free tool
With EPUBCheck, the official W3C validator, and without uploading the file: it all happens in your browser.
Drop an EPUB here
or
Up to 200 MB. The first time, the validator is downloaded (about 3 MB compressed).
EPUBCheck is the official W3C validator, the one publishers and stores use. It checks the file's structure, the package, the documents, internal links and images against the EPUB specification. This is its version 5.4.0, compiled to JavaScript, with its rules unchanged.
The EPUB is read and validated in this tab: the file isn't uploaded. The only download is the validator itself, from pecia.app. Of the book, only its ISBN or which book of our catalogue it is are sent, if you leave the box ticked and it's a published book, counted per month with nothing that identifies you; we also count, anonymously, how many validations are run and their result.