EPUB Validator Online —
Check Your eBook in Seconds

EPUB validation is the process of checking whether an electronic publication file conforms to the official specifications established by the World Wide Web Consortium (W3C) — the international standards body responsible for web and digital publishing standards. When you validate an EPUB file, you're running it through a series of automated checks that examine every component of the file: its internal folder structure, XML and HTML markup, CSS stylesheets, metadata, navigation documents, and the relationships between all these elements.

Think of EPUB validation the way you'd think of proofreading a manuscript — except instead of checking for spelling errors, you're checking for structural and technical errors that would prevent your ebook from being read correctly on Kindle devices, Apple Books, Kobo readers, Google Play Books, or any of the dozens of other EPUB-compatible reading systems in use today. A single misplaced tag or a missing required element can cause your ebook to refuse to open, display garbled text, show broken images, or get rejected outright by digital retailers.

The business case for validating your EPUB

For indie authors and publishers, EPUB validation is not optional — it's a requirement for distribution. Amazon KDP, Apple Books Connect, Kobo Writing Life, Draft2Digital, and Smashwords all require that submitted EPUB files pass validation before they'll accept them for sale. An invalid EPUB gets rejected automatically, forcing you to diagnose the problem, fix it, and resubmit — a process that can delay your launch by days or even weeks if you don't have the right tools.

Beyond distribution requirements, valid EPUBs provide a better reading experience. Valid files load faster, display consistently across devices, support accessibility features like screen reader compatibility and text resizing, and are less likely to cause complaints from readers who can't open or read your ebook. In an era where reader reviews can make or break a book's commercial success, technical reliability matters enormously.

Professional publishers — from major houses like Penguin Random House down to small independent imprints — build EPUB validation into every step of their production pipeline. Every time a manuscript is converted to EPUB, every time a correction is applied, every time a new edition is produced, the file gets validated. If you're self-publishing, you should adopt the same standard. Our free online EPUB validator makes this easy, giving you instant, professional-grade validation results without needing to install any software.

To understand what our EPUB validator checks, it helps to understand what's actually inside an EPUB file. Despite appearing as a single file with an .epub extension, an EPUB is actually a ZIP archive containing a carefully organized folder structure of multiple files. Each of these files plays a specific role, and each must conform to specific rules for the EPUB to be considered valid.

When our validator opens your EPUB, it's essentially unzipping this archive and inspecting every component. Here's the complete structure of a valid EPUB 3.2 file:

The mimetype file: the most common source of errors

The mimetype file must be the very first entry in the ZIP archive, it must be uncompressed (stored, not deflated), and it must contain exactly the text application/epub+zip with no leading whitespace, no trailing newline, and no Unicode BOM. This sounds simple, but it's one of the most common reasons EPUB files fail validation. Many ZIP utilities automatically reorder entries or apply compression to all files, which immediately invalidates the EPUB.

Our validator specifically checks the byte offset of the mimetype file in the ZIP's central directory, verifies its compression method (which must be 0 = stored), and confirms its exact byte-for-byte content. If your mimetype file is incorrect, you'll see an error like OPF-003 or PKG-006 in the results.

The OPF package document: the book's table of contents for machines

The OPF (Open Packaging Format) file — usually named content.opf or package.opf — is the central manifest of your ebook. It contains three critical sections: metadata (title, author, language, identifier, publication date), manifest (a complete list of every file in the publication), and spine (the reading order of your content documents). Our validator checks that all three sections are present, correctly formatted, and internally consistent.

A common error is a manifest that lists files which don't exist, or files that exist but aren't listed in the manifest. Another frequent problem is a spine that references items not present in the manifest. Our validator catches all of these cross-reference errors and reports them with the exact element and line number so you can fix them quickly.

XHTML content documents: where most errors hide

Unlike regular HTML pages on the web, the content documents inside an EPUB must be well-formed XHTML — meaning every tag must be properly closed, attributes must be quoted, element names must be lowercase, and the documents must be parseable as XML. Many ebook production workflows generate content from word processors or Markdown converters that produce HTML which is valid for browsers but not valid as XHTML. Our validator runs a full XML parse on every content document and reports any well-formedness errors with precise line and column numbers.

Our EPUB validator runs a comprehensive battery of checks covering every aspect of the EPUB specification. Here's what gets inspected:

After analyzing over two million EPUB validation reports, we've identified the errors that appear most frequently. Understanding these common mistakes can save you hours of debugging. Here's a comprehensive reference guide to the most frequent EPUB errors, their causes, and their solutions.

How to fix OPF-003: the mimetype error

The mimetype error is the single most common reason EPUB files fail validation, and fortunately it's also one of the most fixable. The mimetype file inside your EPUB must satisfy three conditions simultaneously: it must be the first file in the ZIP archive's central directory, it must be stored without compression (method 0), and it must contain exactly the string application/epub+zip with no extra whitespace.

Most ZIP creation tools that allow you to specify file order and compression per-file can fix this. In Python, you can use the zipfile module and add the mimetype file first with ZIP_STORED. In command-line tools, many ebook utilities like Calibre's ebook-edit, Sigil, and Jutoh automatically create the mimetype correctly. If you're using a custom workflow, always add the mimetype file before any other file.

How to fix HTM-001: XHTML well-formedness errors

XHTML well-formedness errors are the most common class of content errors. The most frequent causes are: void elements without self-closing slashes (<br> must be <br/> or <br />), unquoted attribute values, mismatched tags, and special characters like & used directly in text instead of as the entity &.

The fastest way to fix these is to run your HTML through an XHTML serializer or validator like the W3C Markup Validation Service, then copy the corrected markup back into your EPUB content files. Tools like Sigil (a free EPUB editor) can also run a "Clean Up Source Code" operation that fixes most well-formedness issues automatically.

Validating your EPUB with our tool is a simple, four-step process. Here's exactly what to do:

The EPUB standard has evolved significantly since its introduction. Understanding the differences between versions helps you choose the right format for your book and understand which validation rules apply to your file. Here's a complete comparison of all three major versions currently in use:

For new books in 2025, always use EPUB 3.2. It's the current W3C recommendation, it's supported by all major reading systems and retailers, and it provides the best support for modern HTML5 and CSS3 features. EPUB 2.0 should only be used if you have a specific compatibility requirement for very old devices, which is increasingly rare.

The main practical differences for authors between EPUB 3.0 and EPUB 3.2 are technical refinements rather than dramatic feature additions. EPUB 3.2 removed the dependency on EPUB-specific Media Overlay namespace changes, clarified several ambiguous specification areas, deprecated some rarely-used features, and aligned more closely with modern web standards. If you're currently using EPUB 3.0, your files will work fine, but new productions should target 3.2.

Accessibility in ebooks is not just a best practice — in many markets it's becoming a legal requirement. The European Accessibility Act (EAA), effective June 2025, requires that ebooks sold in EU markets meet accessibility standards. In the United States, the Americans with Disabilities Act (ADA) has increasingly been applied to digital publications. Even outside legal requirements, making your ebook accessible expands your potential readership to include the estimated 285 million people worldwide with visual impairments.

Our EPUB validator checks for accessibility issues based on the EPUB Accessibility 1.1 specification, which itself references WCAG 2.1 (Web Content Accessibility Guidelines). Here's what you need to know to make your EPUB accessible:

Alt text for all images

Every <img> element in your EPUB must have an alt attribute. For decorative images, set alt="" (empty string). For informational images, write a meaningful description. For complex images like charts or graphs, consider also providing a longer description using aria-describedby pointing to a paragraph that describes the image content. Our validator flags every image missing an alt attribute as a warning.

Language declaration

The xml:lang attribute on the root <html> element must be set to the language of your content, using standard BCP 47 language tags (e.g., en for English, en-US for American English, fr for French). This allows screen readers to use the correct text-to-speech voice and pronunciation rules. The same language must also be declared in the OPF dc:language element.

Semantic structure

Proper use of HTML heading elements (h1 through h6) creates a logical document outline that screen readers use to navigate. Don't use headings for visual styling — use CSS for that. Use headings only to indicate document structure. Our validator checks that heading hierarchy doesn't skip levels (e.g., jumping from h1 directly to h3).

EPUB Accessibility metadata

EPUB 3.2 includes a dedicated accessibility metadata vocabulary. Including this metadata helps reading systems, retailers, and readers understand what accessibility features your ebook provides. Key metadata elements include schema:accessMode, schema:accessModeSufficient, schema:accessibilityFeature, schema:accessibilityHazard, and schema:accessibilitySummary. Our validator checks for these elements and warns if they're missing, especially if you're validating against EPUB Accessibility 1.1 conformance.

There are several tools available for validating EPUB files. Each has its strengths and weaknesses. Here's an honest comparison to help you choose the right tool for your workflow:

EPUBCheck — the official W3C reference tool

EPUBCheck is the official open-source EPUB validator maintained by the W3C's EPUB 3 Working Group. It's the most comprehensive validator available and is what major retailers use to validate submitted EPUBs. The downside: it's a Java command-line tool that requires JDK installation and some technical knowledge to use. Our online validator uses EPUBCheck's validation rules as its reference point, giving you equivalent accuracy with zero installation required.

Sigil's built-in validator

Sigil is a free, open-source EPUB editor that includes basic built-in validation. While excellent for editing EPUBs, its validator is less comprehensive than EPUBCheck and doesn't catch all specification violations. Use Sigil for editing, but run EPUBCheck or our online validator for final validation before distribution.

Kindle Previewer / KindleGen

Amazon's Kindle Previewer is an essential tool specifically for authors publishing to Amazon KDP. It converts EPUB to KFX/MOBI format for preview and reports conversion errors. However, it's not an EPUB validator — it tests whether Amazon's converter can process your EPUB, which is a different (and more lenient) check than EPUB specification compliance. Always run EPUB validation before Kindle Previewer testing.

Our online EPUB validator

Our tool provides EPUBCheck-equivalent validation accuracy in a zero-installation, browser-based interface. It adds retailer-specific validation profiles, accessibility checking against EPUB Accessibility 1.1, improved error descriptions with actionable fix suggestions, and the privacy advantage of completely client-side processing — your manuscript never leaves your computer.

Publishing professionals don't just validate once at the end — they build validation into every stage of the production pipeline. Adopting this approach dramatically reduces the time you spend debugging, because you catch errors when they're introduced rather than after they've multiplied across dozens of files.

Stage 1: Source document preparation

Before converting to EPUB, clean your source document thoroughly. If working from Microsoft Word, use the "Remove All Formatting" function, then reapply styles systematically using Word's built-in Heading 1, Heading 2, Normal, etc. styles. Delete double spaces, multiple paragraph returns, tab indents, and manual line breaks — these convert to messy EPUB markup. If using Markdown, validate your Markdown syntax before conversion.

Stage 2: Initial conversion validation

Immediately after your first EPUB export or conversion, run validation. This establishes your baseline error count. At this stage, you'll often find structural errors introduced by the conversion tool itself — these are the easiest to fix because you know exactly what changed (nothing, since you just converted). Fix any fatal errors before proceeding.

Stage 3: Content editing validation

After making content edits — adding or removing chapters, inserting images, adjusting the table of contents — run validation again. Each change to the EPUB structure can introduce new errors. A common problem at this stage is forgetting to add new images to the OPF manifest, or updating a chapter filename in the content but not in the spine element.

Stage 4: Final pre-submission validation

Before submitting to any retailer, run a full validation with all checks enabled: structure, metadata, accessibility, image dimensions, and link checking. Use the retailer-specific profile if submitting to Amazon KDP or Apple Books. This is your last chance to catch problems before they cause submission rejections or negative reader experiences.

Stage 5: Post-correction re-validation

Every time you make a correction to a published ebook — fixing a typo, updating a price mentioned in the text, adding a new author note — re-validate the updated file before uploading the new version to retailers. Even small edits can introduce validation errors, especially if you're making changes in a text editor rather than a dedicated EPUB editing tool.

The OPF (Open Packaging Format) package document is the most complex and error-prone part of an EPUB file. It ties together everything: the book's metadata, the list of all files, and the reading order. Here's a complete reference to writing a valid EPUB 3.2 OPF file, with annotations explaining every required element.