How to Check an EPUB File Someone Else Formatted for You

Run your EPUB through an online validator such as the W3C’s EPUBCheck, check the metadata matches your book’s details, then open the file in at least two different reading apps to spot formatting problems a validator cannot catch. An EPUB that passes technical validation can still have styling errors, broken navigation, or incorrect metadata; catching these before you upload to a retailer saves weeks of back-and-forth.

How Do You Run EPUBCheck Without Installing Java?

EPUBCheck is the official conformance checker for EPUB publications, maintained by the W3C and the DAISY Consortium. It validates EPUB 2 and EPUB 3 files against the official specification, checking that the file is properly assembled, that your book’s details (title, author, ISBN) are correctly recorded, and that all the internal pieces (chapters, images, fonts, links) are present and accounted for.

Your first step should be to ask your formatter whether they ran EPUBCheck and, if so, to share the output log with you. A professional formatter should be validating every file before delivery, and seeing the clean report yourself gives you confidence in the work. If your formatter did not run EPUBCheck (or cannot provide the log), you can run it yourself using one of the free tools below. The visual, navigation, and metadata checks later in this article apply regardless of who ran the validation.

Three reliable online options let you run EPUBCheck without installing anything. Draft2Digital’s Epub Validator runs EPUBCheck on your file and returns a clear pass/fail report. The HMD Publishing validator supports EPUB 2.0, 3.0, and 3.1 files and adds a quality score across structure, compatibility, and accessibility. Pagina EPUB-Checker is a free desktop application for Windows, macOS, and Linux that bundles EPUBCheck behind a graphical interface, so you get a simple window to drag your file into rather than dealing with any technical setup.

One thing to be aware of: online validators typically have file size limits (often in the range of 10 to 30 MB), which is fine for most novels and text-heavy books but may not accommodate heavily illustrated titles. If your EPUB is too large for an online tool, Pagina EPUB-Checker runs on your own computer and handles files of any size.

You can also download and run the official EPUBCheck package directly, but it requires some technical knowledge to install and use because it runs from the command line with no graphical interface. For most authors, one of the three options above is the simplest path.

Something we see regularly at ebookpbook: authors receive an EPUB from a formatter, upload it directly to KDP without checking, and only discover problems when Apple Books or Kobo rejects the same file weeks later. KDP is lenient with EPUB validation errors because it converts your file to its own format; other retailers run stricter checks. Running EPUBCheck first, regardless of where you plan to publish, catches problems that would otherwise surface at the worst possible moment. For a deeper look at why the same file can pass one platform and fail another, see our post on why your EPUB passes Kindle but fails on Apple Books or Google Play.

What Does an EPUBCheck Report Actually Tell You?

EPUBCheck categorises issues into five severity levels: Fatal Error, Error, Warning, Info, and Usage. A Fatal Error prevents the checker from processing the file further. An Error means the file fails to meet a conformance requirement in the EPUB specification. A Warning flags a deviation from recommended practices that may cause rendering problems on some devices.

Not every error matters equally. The most common error indie authors encounter is PKG-026, an incorrect font MIME type declaration. This single error is a frequent reason for rejection by Apple Books, Kobo, and Draft2Digital. Other common issues include a missing or malformed mimetype file, invalid OPF metadata (missing title, language, or identifier elements), manifest items that reference files not present in the EPUB archive, and missing NCX or NAV navigation documents.

If your formatter delivered a file with zero errors and zero warnings, that is a strong technical baseline. If the report shows errors, forward the full report to your formatter; any professional formatter should resolve EPUBCheck errors at no extra charge, since delivering a valid file is the minimum standard.

How Do You Verify Your EPUB’s Metadata Is Correct?

The metadata embedded inside your EPUB should match exactly what you plan to enter on each retailer’s upload form. Mismatches between internal metadata and platform metadata can cause confusion in library catalogues, search results, and AI-powered book recommendation systems.

EPUB files are ZIP archives containing web content and a metadata file called the Package Document (typically named content.opf). This file holds Dublin Core metadata elements including dc:identifier (your book’s unique identifier, often an ISBN or UUID), dc:title, dc:creator (author name), dc:language, dc:publisher, and dc:date. You do not need to read XML to check these fields. Open the EPUB in Calibre (free, available on Windows, macOS, and Linux), select the book, and press E to open the metadata editor. Every field is displayed in a clean interface where you can verify (and correct) the title, author, identifier, language, and publisher.

Pay particular attention to three fields. First, verify the dc:title matches your cover and title page exactly, character for character; KDP’s metadata guidelines require exact matching and prohibit promotional text in title fields. Second, check the dc:identifier field. This is your book’s unique identifier; it can be an ISBN, but it does not have to be. Many platforms (including KDP) do not require an ISBN for ebooks, and your formatter may have used a UUID or other identifier instead. If you do have an ISBN for this ebook edition, make sure it is the correct one: do not use your paperback or hardcover ISBN here. If you distribute through IngramSpark, they require a unique ISBN for each format. Third, confirm dc:language is set correctly (en for English, not left blank or set to a default like und for undefined).

One nuance worth knowing: KDP and IngramSpark both override internal EPUB metadata with the details you enter during upload. The internal metadata still matters because it is what appears in reading apps, library systems, and any retailer that does not override it. Draft2Digital uses your internal metadata as the starting point and shares it between print and ebook editions, so accuracy in the file itself prevents downstream errors.

What Should a Working Table of Contents Look Like?

A properly built EPUB has two navigation structures, and both need to work. The first is the logical TOC: an NCX file in EPUB 2, or an HTML-based Navigation Document (toc.xhtml) in EPUB 3. This is what the reading app uses to generate its built-in navigation menu. The second is an HTML TOC page visible in the book’s content, which readers can scroll to and tap.

To check the logical TOC, open the EPUB in Kindle Previewer or Thorium Reader and use the app’s table of contents panel (usually accessed via a menu icon). Every chapter and section heading should appear, and tapping each entry should jump to the correct location. A common KDP error is “Missing Table of Contents”; this happens when the logical TOC file exists but KDP cannot recognise its structure, often because it references the wrong content document or uses an unsupported navigation format. For a full discussion of ebook TOC requirements, see our post on whether your self-published ebook needs a table of contents.

IngramSpark explicitly states that ebook TOC entries should not reference page numbers, because e-readers display content differently across devices. Apple Books requires a nav element with an epub:type value of “toc” and will flag incomplete or inaccurate TOC structures during ingestion. If your formatter delivered an EPUB 3 file, check that it includes toc.xhtml; if it also includes an NCX file for backward compatibility with older readers, that is a sign of careful work.

How Do You Preview Your EPUB on Different Devices?

Validation tells you the file is structurally correct. Previewing tells you whether it actually looks right. These are different tests, and both are necessary.

You do not need to own every e-reader on the market. Four free tools cover the major ecosystems. Amazon’s Kindle Previewer (Windows and macOS) shows how the file renders on Kindle devices with Enhanced Typesetting enabled; it also flags common conversion issues. Apple Books on any Mac or iOS device previews the EPUB natively; simply double-click the file to open it. Thorium Reader is a free, open-source desktop app for Windows, macOS, and Linux that renders EPUB 3 accurately and includes strong accessibility features. Calibre’s built-in viewer handles both EPUB 2 and 3 and lets you test font overrides, night mode, and different page layouts.

When previewing, do not just skim the first chapter. Scroll through the entire file and check at least three things. First, verify that images are sharp and correctly positioned; blurry or misplaced images are the most visible formatting problem readers notice, and we have covered the technical reasons in our post on why images look blurry in Kindle ebooks. Second, test the file in night mode (dark background, light text). Transparent PNG images that look fine on a white background can become invisible or display dark halos in dark mode. Third, resize the text to both the smallest and largest settings. Reflowable EPUBs should adapt gracefully; if headings overlap body text or images break out of their containers at large sizes, the CSS needs attention.

What Should You Look for When Reading Through the File?

Beyond technical validation and device previewing, a manual read-through catches the formatting problems that tools miss. Treat this as a final quality check, not a line edit.

Work through this checklist systematically:

• Chapter openings: Are chapter titles consistent in font, size, and spacing? Do chapters start on a new page (or screen), or does a chapter start partway down the previous chapter’s last page?
• Paragraph formatting: Is the first paragraph of each chapter flush left (as is conventional) while subsequent paragraphs are indented? Are there any double-spaced gaps between paragraphs that should be single-spaced, or vice versa?
• Special characters: Do smart quotes, em dashes, ellipses, and accented characters render correctly, or have they been replaced with question marks or empty boxes? This is especially common when fonts are not properly embedded.
• Hyperlinks: Do internal links (footnotes, cross-references, TOC entries) jump to the correct location? Do external URLs open correctly?
• Front and back matter: Is the copyright page present with the correct year and ISBN? Does the “About the Author” section appear? If you have a series page or a reader magnet link, verify it is included and functional.
• Cover image: Does the ebook include an internal cover image, and does it match the cover you uploaded separately to the retailer?

Make sure your formatter has the final version of your front cover to include inside the ebook file; it should be the same image you upload separately to the retailer. This is a gap that can appear when the formatter and cover designer are different people: the formatter may be working with an early draft or a low-resolution placeholder, while the final cover only gets uploaded to the platform listing. The cover embedded inside the EPUB is what readers see when they open the book on their device, so if it is blurry or outdated, the first impression suffers even if the retailer’s listing page shows the correct cover.

What Do You Do If Your Formatter Delivered a Bad EPUB?

If your checks reveal problems, document them before contacting your formatter. Take screenshots of rendering issues, copy the full EPUBCheck error log, and note the specific chapter or section where each problem appears. A clear, itemised list gets results faster than a vague “it doesn’t look right.”

Professional formatters expect revision requests; delivering a clean file sometimes takes two or three rounds, particularly for complex layouts with images, footnotes, or embedded fonts. What matters is how they respond. A good formatter will fix EPUBCheck errors without charging extra, because a valid file is the baseline deliverable. Styling adjustments (font choices, spacing, chapter heading designs) may or may not fall under the original scope depending on your agreement.

If you are evaluating whether the problems are serious enough to push back, use this hierarchy. EPUBCheck errors are non-negotiable; the file must pass validation. Metadata inaccuracies (wrong title, missing ISBN) are also non-negotiable. TOC navigation that does not work is a significant problem that affects every reader. Cosmetic styling inconsistencies (slightly different spacing between chapters, a heading that is bold in one place and not in another) are worth flagging but are lower priority. Rendering quirks that only appear in one reading app at one text size may not be fixable without trade-offs elsewhere.

Frequently Asked Questions

Do I need to check my EPUB if the formatter said it passed validation?

Yes. Passing EPUBCheck confirms the file meets the EPUB specification’s structural requirements, but it does not check whether the book looks correct, whether the metadata matches your publishing details, or whether the reading experience is smooth across devices. A file can pass validation and still have misaligned images, broken TOC links, or incorrect author metadata.

Can I check my EPUB on my phone instead of a computer?

You can preview the reading experience on your phone using Apple Books (iOS) or any EPUB-compatible reader app (Android). For validation and metadata inspection, a computer is more practical because online validators and Calibre’s metadata editor work best on desktop browsers. The HMD Publishing validator is mobile-friendly if you need to run a quick check from your phone.

Should I check a reflowable EPUB differently from a fixed-layout EPUB?

Reflowable EPUBs need testing across multiple text sizes and orientations because their layout adapts to the reader’s settings. Fixed-layout EPUBs should be checked at the intended display size to verify that text and images are positioned correctly and that no content is cut off at the edges. Fixed-layout files are also significantly larger, so verify the total file size falls within your target platform’s limits (IngramSpark caps ebook files at 100 MB; individual images must not exceed 5.6 million pixels).

What is the difference between the NCX file and the NAV file in my EPUB?

The NCX (Navigation Control for XML) file is the EPUB 2 navigation format. The NAV file (toc.xhtml) is the EPUB 3 navigation format, built as an HTML document. EPUB 3 files should include a NAV file; including an NCX file alongside it provides backward compatibility with older reading systems that only understand EPUB 2 navigation. If your EPUB 3 file has both, that is thorough work by your formatter.

How many revision rounds should I expect from a professional formatter?

Most professional formatting services include one to three rounds of revisions in their base price. Revisions that fix errors in the formatter’s work (validation failures, incorrect metadata, broken navigation) should always be covered. Revisions that involve changing your mind about design choices (different fonts, different chapter heading styles) may count toward your included rounds or incur additional charges depending on the contract terms.