What does an EPUB validator check?
An EPUB file is a ZIP archive with a strict structure: a mimetype file, a META-INF/container.xml that points to the package file (OPF), and the XHTML chapters, images, CSS and fonts listed in that package. If one piece is wrong, a store can reject the upload or a reader app can show a blank page, a broken table of contents or missing pictures. A validator opens the file and checks each rule before you upload it to Apple Books, Kobo, Google Play Books, Draft2Digital or Amazon KDP.
How to validate an EPUB in 3 steps
- Drop your .epub into the box above. It is opened inside your browser — the file never leaves your computer.
- Read the result: errors (❌) must be fixed, warnings (⚠️) should be fixed, tips (ℹ️) are good practice. Each problem names the file it was found in.
- Fix and re-check. Common packaging problems can be fixed with one click; the tool downloads a repaired copy you can drop in again.
The most common EPUB errors and how to fix them
| Problem | Why it happens | Fix |
|---|---|---|
| mimetype not first or compressed | The EPUB was re-zipped with a normal ZIP tool | Use “Fix & download” — it rebuilds the file with mimetype first and uncompressed |
| Missing dcterms:modified | EPUB 3 needs a last-modified date in the OPF | One-click fix adds the current date |
| Missing dc:language | Exported without a book language | One-click fix adds it — check the code is right (en, ur, ar…) |
| File missing from manifest / manifest file missing | A chapter or image was renamed or deleted | Rename it back or update the <item href> in the OPF |
| Not well-formed XHTML | Unclosed tags such as <br> or <img>, or a stray & | Write <br/>, close every tag, use & |
| Broken link or image | Wrong path or capital letters (Image.JPG vs image.jpg) | Paths in EPUB are case-sensitive — match them exactly |
| No navigation document | EPUB 2 file relabelled as EPUB 3 | Add a nav.xhtml with properties="nav" or regenerate in your editor |
EPUB 2 vs EPUB 3
EPUB 3 is the current standard and is required by most stores for new books. It uses an XHTML navigation document (nav.xhtml) for the table of contents and must include dcterms:modified. EPUB 2 uses an NCX file instead. Many EPUB 3 books also keep an NCX so older readers still show a table of contents. The validator detects the version automatically and applies the right rules.
Do I need to validate an EPUB for Amazon KDP?
KDP accepts EPUB uploads and converts them to its own Kindle format. A clean EPUB converts with fewer problems — broken links, missing images and invalid XHTML are the usual reasons a Kindle preview looks wrong. After this check, open the file in Kindle Previewer to see how it will look on Kindle devices. Your cover should be about 1600 × 2560 px; the validator shows the cover size and links to our ebook cover checker if it needs resizing.
Accessibility checks
Since the European Accessibility Act came into force in June 2025, ebooks sold in the EU are expected to be accessible. The validator flags images without alt text, chapters without a language and missing accessibility metadata (schema:accessMode, schema:accessibilityFeature, schema:accessibilitySummary), so you can fix the basics before submission.
More than a validator
- Book stats: word count, estimated print pages and reading time.
- Reader preview: click any table-of-contents entry to read the chapter and spot formatting problems.
- Book details: title, author, language, identifier, file and image sizes, cover preview.
- Report: download a text report to send to your formatter or co-author.
This is a fast in-browser check of the most common EPUB problems. The official W3C EPUBCheck remains the final authority used by many retailers. Not affiliated with W3C, Amazon, Apple or Kobo.
Why validate an EPUB?
Stores reject EPUB files with structural errors, and small mistakes can make an ebook look broken on some readers. Checking before you upload saves a rejected submission and bad reviews.
What it checks
- The required files: mimetype, META-INF/container.xml and the package (.opf) file.
- Metadata: title, language and a unique identifier.
- Manifest and spine: whether listed files and chapters are consistent.
- XHTML problems such as broken tags.
- Navigation: whether there is a table of contents readers can use.
Errors (❌) must be fixed before uploading; warnings (⚠) are worth fixing but usually accepted.
Try it
Press Try a sample EPUB with mistakes to see what a report looks like, or paste XML from a single file to check it.
Fixing common errors
- Missing language or identifier: add them with the EPUB metadata generator.
- File listed but missing: remove the entry or add the file.
- Broken XHTML: often caused by copy-pasted Word formatting — rebuild with the Word to EPUB formatter.
For final checks before a big release, you can also run the official EPUBCheck tool.