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EPUB Validator Online: Check Your Ebook Before Publishing
An EPUB validator checks your ebook file for structural errors before you upload it to Kindle, Apple Books, or Kobo. Stores reject broken files — and readers' apps crash on them. This guide explains what validation checks, what the common errors mean, and how to fix them with a free online EPUB validator that runs entirely in your browser.
Why validate your EPUB?
An EPUB is a zip file full of XHTML, CSS, images, and metadata. One bad reference — a missing image, a chapter listed in the manifest but absent from the file, a malformed table of contents — and:
- Kindle Direct Publishing may reject the upload or mangle your formatting;
- Apple Books is notoriously strict and bounces invalid files;
- Reader apps can show blank pages or crash on broken navigation.
Validating takes thirty seconds and catches all of it before your readers do.
What does an EPUB validator check?
The industry reference is EPUBCheck, the open-source validator maintained by the publishing industry. A good online EPUB validator covers the same ground:
- Package integrity — is the zip structured correctly, with the mimetype file first and uncompressed?
- Metadata — does the book have a title, a unique identifier, and a language code?
- Manifest vs. files — every file listed in the manifest must exist, and every content file should be listed;
- Spine (reading order) — every chapter referenced must exist and be in order;
- Table of contents — a navigable TOC that actually points at real chapters;
- Content files — each chapter must be valid XHTML; images it references must be included.
Common EPUB errors, explained
Validator output is famously cryptic. Here is what the frequent ones actually mean:
- Missing or compressed mimetype — the file's "ID card" must be the first entry in the zip, stored uncompressed. Some tools get this wrong; repackaging fixes it.
- No unique identifier — stores use this to tell your book apart. Any UUID works.
- Dangling manifest entry — the manifest lists a file that isn't in the zip (often a deleted chapter). Remove the entry.
- Broken spine reference — the reading order points at a chapter ID that doesn't exist. Same fix: remove it.
- Missing image — a chapter references an image that was never included. Either add the image or remove the reference.
- No navigable TOC — readers need a table of contents to jump between chapters. Generate one from your headings.
How to validate (30 seconds)
- Open the free EPUB validator.
- Drop in your .epub file and click Check my EPUB.
- Read the report — every problem is explained in plain English with a suggested fix.
- Click 🔧 Fix on any issue (or Fix all) to repair it automatically, then download the fixed file.
Your file never leaves your browser — validation happens locally on your device, which also means it works on manuscripts you'd rather not upload anywhere.
What to do after validation passes
A clean report means your EPUB is structurally sound. Next steps before publishing:
- Check your metadata and cover — title, author, description, and a proper cover image;
- Preview the book in Kindle Previewer and Apple Books;
- If you're also doing a paperback, export a print-ready PDF with the right trim size.
Questions
- Is there a free EPUB validator online?
- Yes — KindleReady's validator is free, needs no account, and even fixes problems automatically. The desktop alternative is the open-source EPUBCheck tool.
- Will Kindle accept my EPUB without validation?
- Kindle is forgiving, but Apple Books and Kobo are not. Validating once protects you on every store.
- What's the difference between errors and warnings?
- Errors can break readers or get your file rejected — fix all of them. Warnings (like a missing cover) are recommendations, not blockers.
Ready? Validate your EPUB now →