Validate your EPUB files instantly for structure, metadata, and compliance with EPUB 2.0 and 3.0 standards. Get detailed validation results with error reporting — no signup required.
Click to upload or drag and drop
EPUB files only (max 200 MB)
EPUB validation is the process of checking an ebook file against the official EPUB specification to ensure it is correctly structured and will render properly across all reading devices and platforms. An EPUB validator examines the internal file structure, required metadata, content manifest, reading spine, and navigation documents to identify errors, missing components, and compliance issues before you publish.
The EPUB format is maintained by the W3C (World Wide Web Consortium), which took over stewardship from the IDPF (International Digital Publishing Forum) in 2017. The specification defines strict requirements for how ebook files must be packaged, what metadata must be present, and how content documents reference each other. When an EPUB file violates these requirements, reading apps may display the book incorrectly — or the file may be rejected entirely by ebook retailers.
For self-publishing authors and small publishers, EPUB validation is a critical quality-control step. It catches problems that are invisible when previewing a book in a single app but cause failures across the wider ecosystem of devices, apps, and distribution platforms. Running your file through an EPUB checker before submission saves time, avoids retailer rejections, and ensures readers get a professional experience.
Several major ebook platforms enforce strict EPUB validation and will reject files that contain errors. Apple Books is particularly strict, requiring valid EPUB 3.0 files and rejecting many common structural issues. Kobo, Barnes & Noble Press, and Google Play Books all run validation checks on uploaded files. Aggregators like Smashwords and Draft2Digital use EPUBCheck internally and reject files that fail validation.
Amazon KDP accepts EPUB uploads and converts them to its proprietary KFX and AZW formats. While KDP doesn't enforce EPUB validation directly, invalid files often produce poor conversion results — broken tables of contents, missing images, incorrect formatting, and lost metadata. Starting with a validated EPUB consistently produces better Kindle output.
Beyond retailer requirements, invalid EPUBs create a poor reader experience. Common symptoms include broken table of contents navigation, missing or misplaced images, incorrect chapter ordering, and rendering inconsistencies across different reading apps. Professional publishing houses validate every EPUB before release as a standard quality-control step, and independent authors should follow the same practice.
Our EPUB file checker performs three tiers of validation and produces quality scores from 0 to 100 for each category, giving you a clear, at-a-glance view of your ebook's compliance status.
The validator first checks the fundamental EPUB package structure. It verifies that the file is a valid ZIP archive, that the mimetype file exists as the first entry and contains exactly application/epub+zip, that the META-INF/container.xml file is present and well-formed, and that the container correctly references an OPF (Open Packaging Format) file within the package.
The tool parses the OPF file and checks for all required metadata fields: title, author/creator, language, and unique identifier. It validates the manifest (the list of all files in the ebook), the spine (the reading order), and the navigation document — NCX for EPUB 2.0 or NAV for EPUB 3.0. The EPUB version is auto-detected and version-specific rules are applied.
Finally, the validator checks that every file referenced in the manifest actually exists within the EPUB ZIP package. It verifies internal file references, detects encryption, and identifies orphaned files. This tier catches the common problem of OPF manifests referencing images or stylesheets that were moved or renamed without updating the references.
Our validator is designed for fast, practical checks. For comprehensive spec-level validation including CSS parsing, font obfuscation checks, and accessibility auditing, we recommend also using the W3C EPUBCheck tool.
EPUB 2.0 was the dominant ebook format for years and remains widely used in backlist titles. It uses the NCX (Navigation Control for XML) file for table of contents navigation, supports XHTML 1.1 and DTBook for content documents, and has limited multimedia capabilities. EPUB 2.0 does not natively support audio, video, or scripted interactivity.
EPUB 3.0 is the current standard and a significant upgrade. It replaces the NCX with an HTML5-based NAV document for navigation, uses XHTML5 for content, and adds native support for embedded audio and video, MathML, SVG graphics, and JavaScript. EPUB 3.0 also introduces much stronger accessibility features, including semantic markup and ARIA roles, making it the preferred format for accessible publishing.
Most new ebooks should use EPUB 3.0, and all major retailers and reading apps support it. However, many older titles and some conversion tools still produce EPUB 2.0 files. Our EPUB validator automatically detects the version of your file and applies the correct validation rules for either standard, so you don't need to know the version in advance.
These are the most frequent issues our EPUB error checker detects, along with practical guidance for resolving each one.
PKG-026 is the single most common reason for EPUB rejection by distributors. This error means a font file in your EPUB has an incorrect MIME type in the OPF manifest — for example, fonts declared as application/x-font-ttf or application/x-font-otf instead of the correct font/sfnt (for TTF/OTF) or application/font-woff (for WOFF). Apple Books, Kobo, Draft2Digital, and Smashwords all reject EPUBs with PKG-026 errors. Our auto-fix feature corrects all font media types automatically with one click.
The mimetype file must be the first entry in the ZIP archive and contain exactly application/epub+zip with no trailing newline, whitespace, or byte-order mark. If this file is missing or malformed, most reading systems and retailers will reject the EPUB outright. To fix it, re-create the EPUB ensuring the mimetype is added first without compression.
The OPF (content.opf) file must include a <dc:title>, <dc:language>, and <dc:identifier> at minimum. EPUB 3.0 additionally requires a <meta property="dcterms:modified"> timestamp. Missing or empty values for these fields will trigger validation errors. Open the OPF in a text editor or use Sigil to add the required metadata.
This error occurs when the OPF manifest references files that don't exist in the EPUB package — usually due to renamed or moved files, incorrect paths, or URL-encoding mismatches. Check that every href in your manifest items points to a real file inside the EPUB, and that paths are case-sensitive and correctly encoded.
EPUB 2.0 files require an NCX (toc.ncx) file for navigation, and EPUB 3.0 files require a NAV document (an XHTML file with properties="nav" in the manifest). Without a valid navigation document, the table of contents won't work in reading apps. Sigil and Calibre both have built-in tools to generate navigation documents from your heading structure.
Some EPUBs use font obfuscation (IDPF or Adobe methods) to protect embedded fonts. If the obfuscation metadata is malformed or the encryption.xml file references incorrect algorithms, you may see warnings. This primarily affects EPUBs from professional publishing pipelines and can usually be resolved by re-exporting from your publishing tool with correct font embedding settings.
EPUB content documents must be valid XHTML — every tag must be properly closed, attributes quoted, and special characters escaped as entities. Common problems include unclosed <br> tags (should be <br />), unescaped ampersands, and missing XML declarations. Our AI-powered repair feature can fix many XHTML well-formedness issues automatically.
Each major ebook distributor handles EPUB validation differently. Understanding their requirements helps you prepare files that pass on the first submission.
Amazon KDP accepts EPUB uploads and converts them to AZW and KFX formats for Kindle devices. KDP does not enforce strict EPUB validation, but clean, valid files produce significantly better conversions with fewer formatting issues.
Apple Books has the strictest requirements among major retailers. It requires valid EPUB 3.0 files and rejects many common errors that other platforms might accept. Testing with our EPUB validator before uploading to Apple Books can save multiple rounds of rejection and resubmission.
Kobo requires valid EPUB files for both direct uploads through Kobo Writing Life and for titles distributed through Kobo Plus. Barnes & Noble Press validates EPUBs before publishing to the Nook platform. Google Play Books accepts EPUBs with moderate validation, though severely malformed files will be rejected.
Smashwords and Draft2Digital, two popular aggregators for wide distribution, run EPUBCheck on every uploaded file and reject those that fail validation. IngramSpark, used for wide distribution to bookstores and libraries, also requires valid EPUB files and applies its own quality checks during the upload process.
Our online EPUB validator and the W3C EPUBCheck tool serve complementary purposes. Understanding when to use each will give you the best validation workflow.
Our EPUB validator runs entirely in the browser with no installation required. It provides instant visual results, extracts and displays your book's metadata, and generates downloadable reports. It's ideal for quick checks during editing, metadata review, and pre-submission validation. You can use it on any device including phones and tablets.
W3C EPUBCheck is the official reference validation tool, available as a command-line application and a Java library. It performs the deepest possible validation against the full EPUB specification, including content document parsing, CSS validation, font checking, and accessibility auditing. EPUBCheck is the industry standard for final production validation and is what most distributors use internally.
When to use our tool: During your editing and formatting workflow for quick checks, when you want to inspect metadata, and before uploading to any retailer. When to use EPUBCheck: For final production validation, accessibility certification, and when a distributor specifically requires EPUBCheck compliance. You can access EPUBCheck at check.w3.org/epub.
The best way to avoid EPUB validation errors is to use a reliable tool for creating your ebook in the first place. Here are the most popular options used by authors and publishers.
Sigil is a free, open-source EPUB editor available for Windows, Mac, and Linux. It provides direct access to the EPUB's internal files and has built-in validation, making it excellent for fixing structural issues. Calibre is a free ebook management application that includes format conversion and basic EPUB editing — useful for converting between formats and batch-processing files.
Adobe InDesign is the professional standard for book layout and includes EPUB export functionality. It produces high-quality EPUBs but requires careful export settings to avoid validation issues. Vellum is a Mac-only tool popular with self-publishing authors for its beautiful templates and reliable EPUB output.
If you need professional help creating or fixing your EPUB files, HMD Publishing offers book formatting services that include EPUB creation, validation, and retailer-ready file delivery.
Explore our other free tools to help with your publishing workflow