HMD Publishing - Professional Book Publishing Services
About HMD Publishing
HMD Publishing is a leading professional book publishing services company based in the UK, serving authors worldwide since 2015. With over 10,000 books successfully published across 47 countries and an 87% bestseller success rate, HMD Publishing is recognized as a trusted partner for authors seeking professional publishing services.
Validate your EPUB against UK ebook distribution requirements. Check compatibility with Amazon KDP UK, Apple Books, Kobo, and other UK platforms before uploading.
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EPUB Validation for UK Distribution
The UK ebook market is the second-largest English-language market globally. Amazon dominates with Amazon.co.uk, but Apple Books, Kobo, and Google Play Books hold significant market share. UK readers are also heavy users of library ebook services through BorrowBox and Libby (OverDrive), making EPUB quality especially important.
UK-specific EPUB considerations include proper language metadata (use 'en-GB' not 'en-US' for British English content), correct currency formatting in any pricing references, and adherence to UK spelling conventions in metadata fields. Apple Books' UK store has identical technical requirements to the US store but indexes metadata separately.
For VAT purposes, UK ebook prices include VAT at 0% (zero-rated since January 2025). Your EPUB metadata should include the correct ISBN registered with Nielsen, the UK ISBN agency. If you're pricing differently for the UK market, ensure your metadata and distribution settings are consistent across platforms.
The British Library's legal deposit scheme now includes ebooks. UK publishers must deposit digital publications including EPUBs. The Non-Print Legal Deposit Regulations 2013 require a copy of every UK-published ebook to be made available to the British Library and five other legal deposit libraries. Having a valid, well-structured EPUB simplifies this legal requirement.
Kobo has a particularly strong presence in the UK market and is the primary ebook platform for WHSmith. Kobo's ePub quality check is among the strictest — they validate against EPUB 3.x standards and reject files with CSS errors, broken internal links, or missing table of contents. Run your file through our validator before submitting to Kobo.
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Frequently Asked Questions
What is an EPUB validator?
An EPUB validator is a tool that checks your ebook file against the official EPUB specification maintained by the W3C. It verifies that the file structure, metadata, manifest, spine, and content references are all correct and compliant. Validation ensures your ebook will display properly across all reading devices and won't be rejected by retailers.
How do I validate my EPUB file?
Upload your EPUB file by dragging it onto the validator or clicking to browse. The tool automatically checks your file's structure, metadata, and content references against EPUB 2.0 and 3.0 standards. Within seconds, you'll see colour-coded results showing any errors, warnings, and extracted metadata. No signup or installation is required.
Is this EPUB validator free?
Yes, this EPUB validator is completely free to use with no signup, no account creation, and no usage limits. You can validate as many EPUB files as you need. The tool runs in your browser and returns instant results.
What EPUB versions does this tool support?
Our validator supports EPUB 2.0, EPUB 3.0, and EPUB 3.1 formats. The tool automatically detects your EPUB version from the package metadata and applies the appropriate validation rules. EPUB 3.x files with features like media overlays and embedded fonts are fully supported.
What errors can an EPUB validator detect?
Our validator detects three categories of errors: structural issues (missing mimetype file, invalid ZIP format, broken container.xml), metadata problems (missing title, language, or identifier, malformed OPF), and content errors (manifest files not found, broken internal references, missing navigation documents). Each error includes a specific message to help you fix it.
What's the difference between EPUB 2.0 and EPUB 3.0?
EPUB 2.0 uses NCX for navigation and XHTML 1.1 for content, with limited multimedia support. EPUB 3.0, the current standard, uses an HTML5-based NAV document for navigation and supports audio, video, MathML, SVG, and JavaScript. EPUB 3.0 also has significantly better accessibility features. Most new ebooks should use EPUB 3.0, though many older titles still use 2.0.
Do I need to validate my EPUB before uploading to Amazon KDP?
While Amazon KDP converts EPUB files to their proprietary KFX/AZW format internally, uploading a valid, error-free EPUB produces significantly better conversion results. Invalid EPUBs can lead to formatting problems, broken tables of contents, and missing images in the final Kindle version. Validating before upload saves time and avoids rejection or poor rendering.
What doesn't this tool validate?
Our validator focuses on structural and metadata compliance for fast, practical checks. It does not validate CSS styling rules, JavaScript execution, font obfuscation, advanced accessibility features (WCAG/ARIA), or DRM encryption. For comprehensive spec-level validation including these features, use the W3C EPUBCheck tool (https://check.w3.org/epub/) as a complement.
How do I fix common EPUB errors?
The most common fixes include: ensuring the mimetype file contains exactly 'application/epub+zip' with no extra whitespace, verifying META-INF/container.xml points to a valid OPF file, adding required metadata fields (title, language, identifier), and checking that all files listed in the manifest exist in the EPUB package. Tools like Sigil (free, open-source) and Calibre can help fix structural issues.
Can I validate EPUB files on my phone?
Yes. Our EPUB validator is fully mobile-friendly and works on any device with a modern web browser, including iPhones, Android phones, and tablets. Simply visit the page, upload your EPUB file, and get instant validation results — no app installation needed.
Is my EPUB file kept on your servers?
No. Your EPUB file is processed in memory during validation and is never permanently stored on our servers. The validation happens instantly, results are returned to your browser, and the file data is immediately discarded. We do not retain, share, or analyse any uploaded content.
What is the maximum file size for validation?
The maximum upload size is 200MB. Most standard EPUBs are well under this limit — text-heavy books are typically under 1MB, and image-heavy books rarely exceed 5MB. If your file is larger, it likely contains uncompressed images that should be optimised. Files are also checked against ZIP bomb protection with a 500MB uncompressed limit.
How does this compare to EPUBCheck?
Our tool is browser-based, requires no installation, and gives instant visual results with metadata extraction — ideal for quick checks during your workflow. EPUBCheck is a command-line tool that performs deeper spec-level validation including font checking, accessibility auditing, and full content parsing. Use our tool for routine validation and EPUBCheck for final production checks before submitting to distributors.
Which ebook platforms require valid EPUBs?
Apple Books enforces strict EPUB 3.0 validation and rejects many errors. Kobo, Barnes & Noble Press, and Google Play Books all require valid EPUBs. Smashwords and Draft2Digital run EPUBCheck on uploads and reject invalid files. Amazon KDP accepts EPUBs but converts them internally, so valid files produce better results. IngramSpark requires valid EPUBs for wide distribution.
Can I download the validation results?
Yes. After validation completes, you can download a detailed text report of all results including errors, warnings, metadata, and file structure information. This report is useful for sharing with your ebook formatter or keeping as a record of your file's compliance status before publishing.
What do the EPUB quality scores mean?
After validation, your EPUB receives three quality scores from 0 to 100: Structure (package integrity, metadata, and navigation), Compatibility (distributor acceptance, font types, and URLs), and Accessibility (screen reader support, metadata, and ARIA roles). These are combined into a weighted overall score — Structure and Compatibility each count for 40%, and Accessibility counts for 20%. A score of 80 or above (green) means your file is in good shape. 50–79 (amber) means there are issues worth fixing. Below 50 (red) indicates serious problems that will likely cause rejection by distributors.
What does the EPUB auto-fix repair?
The auto-fix handles the errors most likely to cause retailer rejection: PKG-026 font media type errors, missing or malformed mimetype files, incomplete OPF metadata (title, language, identifier), broken manifest references, missing navigation documents, empty links that affect accessibility, and accessibility metadata gaps. After fixing, the tool re-validates your EPUB and shows before-and-after quality scores so you can see exactly how much your file improved. Some issues — such as corrupted content files or deeply malformed XML — may still require manual editing in Sigil.
Can this tool fix EPUB errors automatically?
Yes. When our validator detects errors in your EPUB file, it automatically analyses which errors can be fixed and offers an auto-fix option for $12.99. The fixer can repair missing or malformed mimetype files, add required metadata fields, correct broken manifest references, and fix other common structural issues. After fixing, the tool re-validates the EPUB and lets you download the corrected file.
What EPUB errors can the auto-fixer repair?
The auto-fixer handles the most common EPUB errors that cause retailer rejections: missing or incorrect mimetype files, incomplete OPF metadata (missing title, language, or identifier), broken manifest references where files have been moved or renamed, missing navigation documents, and malformed container.xml. After applying fixes, the tool re-validates to confirm all issues are resolved. Some errors — such as corrupted content files or deeply malformed XML — may require manual editing in a tool like Sigil.
How much does the EPUB fixer cost?
EPUB validation is completely free with no limits. The auto-fix feature costs $12.99 per file, paid securely via Stripe. This includes automatic error repair, post-fix re-validation, and a 24-hour download window for the fixed EPUB file. A download link is also emailed to you for convenience.
What is a PKG-026 error?
PKG-026 is an EPUBCheck error code meaning a font file in your EPUB has an incorrect media type. For example, fonts declared as 'application/x-font-ttf' instead of the correct 'font/sfnt'. PKG-026 is the most common cause of EPUB rejection by distributors including Apple Books, Kobo, Draft2Digital, and Smashwords. Our tool detects PKG-026 errors and fixes them automatically with one click.
Which distributors check EPUB compliance?
Apple Books enforces the strictest EPUB 3.0 validation. Kobo, Barnes & Noble Press, and Google Play Books all require valid EPUBs. Smashwords and Draft2Digital run EPUBCheck on every upload and reject files that fail. IngramSpark requires valid EPUBs for wide distribution to bookstores and libraries. Amazon KDP accepts EPUBs but converts them internally — valid files produce significantly better Kindle conversions. Tolino and OverDrive also validate before accepting files.
What Is EPUB Validation?
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.
Why Validate Your EPUB Before Publishing?
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.
What Our EPUB Validator Checks
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.
Structure Validation
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.
Metadata Validation
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.
Content Validation
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.
Automatic EPUB Error Fixing
After validation, HMD Publishing's EPUB Fixer can automatically repair the most common errors that cause distributor rejection. For £9.99 (approx. $12.99 USD), the fixer repairs PKG-026 font media type errors, malformed or missing mimetype files, broken manifest references, missing or invalid OPF metadata, and XHTML well-formedness issues. The fix engine processes your file server-side and returns a corrected EPUB ready for re-upload to Apple Books, Kobo, IngramSpark, and other distributors — no manual editing required. After fixing, the tool re-validates your EPUB and shows before-and-after quality scores so you can see exactly what improved.
EPUB 2.0 vs EPUB 3.0: Key Differences
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.
Common EPUB Errors and How to Fix Them
These are the most frequent issues our EPUB error checker detects, along with practical guidance for resolving each one.
PKG-026: Font Media Type Error
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.
Missing or Incorrect Mimetype File
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.
Invalid OPF Metadata
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.
Manifest Items Not Found
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.
Missing NCX or NAV Document
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.
Font Obfuscation Errors
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.
XHTML Declaration and Well-Formedness Issues
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.
EPUB Validation Requirements by Platform
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.
EPUB Validator vs EPUBCheck
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.
Tools for Creating Valid EPUBs
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.
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