EPUB validator

Why validate before you publish

Retailers run this check on submission. Finding out from a rejection email costs you a day and tells you very little — the store reports that the file failed, not which line of which chapter did it.

Running it yourself first turns that into a list you can act on, with the file path and line number attached to each problem. It is also worth doing on an EPUB produced by any other tool: a converter that reported success can still emit a file that fails the specification.

What we do with your file

It is read into memory, written to a temporary path so the validator can open it, checked, and deleted. Nothing is written to a database and nothing lands in storage. That is the whole lifecycle — see the privacy policy for how the rest of the site behaves.

Questions

What is EPUBCheck?

The reference conformance checker for EPUB, maintained by the DAISY Consortium on behalf of the W3C. It is the tool the standard is checked against, and the one the retailers run on submission — so passing it here is the closest you can get to knowing a store will accept the file.

Is this different from the official tool?

It is the official tool. EPUBCheck ships as a Java command-line program; this page runs that program on your file and formats what it says. The value we add is not having to install Java and learn a CLI to answer one question.

Do you keep my book?

No. The file is held in memory for the length of one request, passed to the validator, and discarded. There is no database row, no stored copy, and no link to come back to — reload the page and the result is gone.

What's the difference between an error and a warning?

An error is a breach of the specification and can get a file rejected outright. A warning flags something legal but likely unintended — a missing accessibility attribute, an unusual structure. Warnings rarely block publication, but they often point at something that will look wrong on a device.

It found dozens of errors. Where do I start?

Usually at the top, and usually there is one cause. A long list is typically one structural problem repeated across every chapter — a broken navigation document, or a stylesheet the package never declared. Fixing the root normally clears the list.

My file passes. Am I definitely fine?

Valid means well-formed, not well-made. EPUBCheck cannot tell you the chapter breaks land in the right places or that the text reads properly on a small screen. It rules out the category of problem that gets a file rejected, which is worth a lot and is not everything.