You validate an EPUB file by running it through EPUBCheck, the official W3C conformance checker, which scans the file’s internal structure, metadata, navigation, CSS, and image references against the full EPUB specification. Free alternatives include Sigil’s built-in validation, Calibre’s Check Book tool, and browser-based validators like the HMD Publishing EPUB Validator — all of which catch the most common errors that cause retailer rejections before you ever hit “upload.”
- Why Does EPUB Validation Matter Before Uploading?
- EPUBCheck: The Official W3C Conformance Checker
- Online EPUB Validators for Quick Browser-Based Checks
- How to Validate an EPUB in Sigil
- Calibre’s Check Book Tool for Finding EPUB Errors
- The Most Common EPUB Validation Errors and How to Fix Them
- Frequently Asked Questions
Why Does EPUB Validation Matter Before Uploading?
An EPUB file is a ZIP archive with a precisely defined internal structure, and any deviation from the specification can cause outright rejection or rendering failures on reader devices. Retailers like Amazon KDP, IngramSpark, Apple Books, and Kobo all run their own ingestion checks, and a file that fails validation may be rejected silently or — worse — accepted but displayed incorrectly on certain e-readers.
Catching errors before submission saves time and prevents the frustrating cycle of uploading, waiting for processing, receiving a vague rejection email, guessing at the problem, and re-uploading. A clean validation pass on your end means the file that reaches readers is the file you intended to publish.
If you’re working with EPUB 2 or EPUB 3 files, the validation requirements differ slightly — EPUB 3 uses a NAV document for navigation while EPUB 2 relies on an NCX file — but both versions are fully supported by the tools covered below.
EPUBCheck: The Official W3C Conformance Checker
EPUBCheck is the gold standard for EPUB validation. It is the official W3C conformance checker, maintained by the DAISY Consortium under an MIT open-source license, and it is the same tool that most distributors use internally to verify incoming files.
EPUBCheck runs as a command-line Java application. After installing Java, you download the latest release from the EPUBCheck GitHub repository, then run it against your file:
java -jar epubcheck.jar yourbook.epubThe tool evaluates your file against the full EPUB specification, including CSS parsing, font obfuscation checks, and — in recent versions — accessibility auditing against WCAG guidelines. It reports errors (which must be fixed) and warnings (which should be reviewed) with specific file paths and line numbers, making it straightforward to locate and correct problems.
EPUBCheck supports both EPUB 2 and EPUB 3 publications. If you are unsure which version your file uses, EPUBCheck detects it automatically. For authors who find the command line intimidating, several of the tools below wrap EPUBCheck in a friendlier interface.
Online EPUB Validators for Quick Browser-Based Checks
If you want a quick check without installing anything, browser-based validators are the fastest option. The HMD Publishing EPUB Validator and Draft2Digital’s EPUB validator both accept file uploads directly in the browser and return validation results within seconds.
Online validators are ideal for iterative checking during the editing process — upload, review errors, fix them in your editor, upload again. They typically run EPUBCheck or a comparable engine on their server, so the results are consistent with what retailers will see.
The trade-off is that you are uploading your manuscript to a third-party server. For most self-published authors this is not a concern (you will be publishing the book publicly anyway), but if you are working with sensitive pre-release material, a local tool like EPUBCheck or Sigil keeps the file on your own machine.
How to Validate an EPUB in Sigil
Sigil is a free, open-source EPUB editor available for Windows, Mac, and Linux. Beyond editing, it includes a built-in well-formedness check that catches structural XML errors in your EPUB’s content documents.
For deeper validation, Sigil supports the FlightCrew plugin (available from the Sigil-Ebook GitHub repository), which performs EPUB 2 conformance checking directly within the editor. The advantage of validating inside Sigil is that you can fix errors immediately — broken internal links, missing metadata, malformed HTML — without switching between tools.
Sigil also provides a visual view of the EPUB’s internal file structure (the OPF manifest, spine order, and NCX or NAV document), which helps you understand what the validator is flagging and why.
Calibre’s Check Book Tool for Finding EPUB Errors
Calibre is a free e-book management application that many self-published authors already have installed. Its built-in “Check Book” feature (accessible via Tools → Check Book in the Calibre editor) detects malformed HTML, invalid or unknown CSS styles, broken internal links, unreferenced files, and common OPF packaging problems such as duplicate spine or manifest items.
Calibre’s Check Book is not a full EPUBCheck replacement — it focuses on the errors most likely to cause rendering problems rather than strict spec conformance. But for a quick pre-flight check, especially if you are already using Calibre to manage your e-book library or convert between formats, it catches the majority of issues that would cause problems on real devices.
One practical workflow is to use Calibre’s Check Book for rapid iteration during editing, then run a final EPUBCheck pass before uploading to retailers. This gives you the speed of a GUI tool during development and the thoroughness of the official validator for your final file.
The Most Common EPUB Validation Errors and How to Fix Them
Certain errors appear in almost every first-time EPUB validation. Knowing what to expect makes them faster to resolve.
Metadata errors
Every EPUB must include a title, language, and unique identifier in its OPF file. Missing or malformed metadata is one of the most frequent validation failures. If you used a conversion tool that stripped metadata, you can add it back in Sigil’s Metadata Editor or directly in the OPF XML.
Image format problems
EPUB images must be in RGB colour mode using JPEG or PNG format. CMYK images (common in files repurposed from print layouts), TIFF files, and WebP images will trigger validation errors because most e-readers cannot display them. Convert problem images to RGB JPEG or PNG before embedding them. If you are working with PNG images that need transparency, ensure they are in RGB mode rather than CMYK.
Navigation and spine errors
The spine element in the OPF file defines the reading order, and the NCX (EPUB 2) or NAV document (EPUB 3) must accurately reflect it. Mismatches — such as content files listed in the manifest but missing from the spine, or table-of-contents entries pointing to nonexistent anchors — are common after restructuring a book’s chapters.
Broken internal links and duplicate IDs
Cross-references between chapters, footnotes, and endnotes rely on internal hyperlinks. If you renamed or moved content files, those links may be broken. Similarly, duplicate element IDs within a single XHTML file (or across files in EPUB 3) cause validation failures and can confuse e-reader navigation.
CSS issues
Invalid or unsupported CSS properties appear frequently in EPUBs converted from Word or InDesign. E-readers support a limited subset of CSS, and properties like position: absolute or complex selectors may be flagged. Simplifying your stylesheet to standard properties — font, margin, padding, text-align, line-height — resolves most CSS warnings.
Frequently Asked Questions
Do I need to validate my EPUB if I’m only uploading to Amazon KDP?
Yes. Amazon KDP runs its own ingestion checks, and files with structural errors may be rejected or display incorrectly on Kindle devices. Validating before upload catches problems that KDP’s error messages often describe only vaguely, saving you multiple re-upload cycles.
Is EPUBCheck free to use?
EPUBCheck is completely free and open source under the MIT license. It is maintained by the DAISY Consortium under the W3C and can be downloaded from the official GitHub repository at no cost.
Can I validate an EPUB on my phone or tablet?
Browser-based validators like the HMD Publishing EPUB Validator work on any device with a web browser, including phones and tablets. You upload your EPUB file through the browser and receive validation results without installing any software.
What is the difference between an error and a warning in EPUBCheck?
Errors indicate violations of the EPUB specification that must be fixed — retailers may reject files with errors. Warnings flag potential issues that do not strictly violate the spec but could cause rendering problems on certain devices. Fix all errors before uploading; review warnings and address any that affect your book’s appearance.
How often should I validate during the EPUB creation process?
Validate early and often. Run a quick check after each major editing pass — adding chapters, embedding images, restructuring navigation — rather than waiting until the final file is complete. Catching errors incrementally is far easier than debugging a file with dozens of issues at the end.
Validating your EPUB before uploading is one of the simplest steps you can take to ensure a clean submission and a consistent reading experience across every device and retailer. The tools are free, the process takes minutes, and the alternative — rejected uploads and broken formatting on reader devices — costs far more in time and frustration.
