Ebook Formatting Requirements for Self-Published Authors

Ebook formatting is one of the most practically misunderstood aspects of self-publishing. Many authors upload a Word document, see that it 'worked,' and assume formatting is solved. But the difference between a book that passes platform review and a book that displays beautifully across all devices and reading apps — on Kindle, Kobo, Apple Books, and every other platform — is the difference between a converted file and a properly formatted one.

This guide covers what ebook formatting actually is, what file formats each platform accepts and prefers, what the ePub standard requires, how formatting tools handle the technical work, common problems that cause platform rejection or poor display, and how to validate your file before submitting anywhere.

The Ebook File Formats: ePub, MOBI, and KF8

There are two primary ebook file formats you need to understand as a self-publisher.

ePub

ePub (.epub) is the open industry standard ebook format, maintained by the World Wide Web Consortium (W3C) and the International Digital Publishing Forum (IDPF). It is the preferred or required format for every wide platform: Kobo, Apple Books, Barnes and Noble Press, Google Play Books, and Draft2Digital. An ePub file is essentially a zipped package containing HTML files (the book's chapters), CSS stylesheets (the formatting), images, and a set of XML files that describe the book's structure and metadata.

ePub comes in two main versions: ePub 2 and ePub 3. ePub 2 is widely supported across all platforms and is the safer choice for broad compatibility. ePub 3 supports more advanced features (enhanced navigation, audio and video for enhanced ebooks, MathML for technical content) but is not required for standard prose ebooks. All major wide platforms accept both versions.

MOBI / KF8 / AZW3

MOBI is Amazon's historical ebook format. Its successor is KF8 (Kindle Format 8), which is the format Amazon's Kindle devices and apps actually use. Amazon KDP accepts ePub files and converts them to KF8 internally — you do not need to produce a MOBI or KF8 file yourself. Amazon also directly accepts .mobi files if you already have them, but since ePub produces better conversion results and is universally supported, ePub is the recommended starting point for all ebook production, including for KDP.

What Each Major Platform Accepts

The practical takeaway: produce an ePub file and you can publish on every platform. Produce only a .docx Word document and you are relying on each platform's conversion quality, which varies significantly.

What's Inside an ePub File

Understanding ePub structure helps you understand why formatting problems occur and how to fix them. An ePub is a ZIP archive with a specific internal structure:

• mimetype — a plain text file identifying the file as application/epub+zip

• META-INF/container.xml — tells ePub readers where to find the content

• OEBPS/ (or similar) — the content directory, containing:

• content.opf — the package document: lists every file in the ePub and provides the book's metadata (title, author, language, ISBN, etc.)

• toc.ncx (ePub 2) or nav.xhtml (ePub 3) — the table of contents

• Chapter HTML files — the actual book content as XHTML

• stylesheet.css — the CSS that controls typography, spacing, indents, and visual styling

• images/ — the cover and any images used in the book

When a formatting tool like Vellum or Atticus produces an ePub, it is generating all of these files automatically. When a platform's conversion engine processes your Word document, it is attempting to create these files from your Word formatting — with varying results.

Formatting Tools: What They Do and Which to Use

Vellum (Mac only)

Vellum is the most widely recommended ebook formatting tool in the indie author community. It produces beautiful, Apple Books-compliant ePub and KDP-compatible files from your manuscript. You import your Word document or paste in your text, choose a book style and design, configure chapter breaks and front/back matter, and Vellum generates ePub files optimized for each major platform alongside a print-ready PDF. Vellum handles all ePub structure, CSS, and platform-specific optimizations internally — you don't need to understand ePub to use it.

Vellum is a paid application (one-time purchase or subscription) and is Mac-only. For most fiction and standard nonfiction, it produces the best consumer-facing ebook output available without professional design involvement.

Atticus (Mac and Windows)

Atticus is the cross-platform alternative to Vellum. It handles both ebook and print formatting, accepts Word documents, and produces ePub and print PDF outputs with quality comparable to Vellum. It is subscription-based. The interface is slightly less polished than Vellum's, but the output quality is excellent and it correctly handles wide platform ePub requirements. For Windows users, Atticus is the recommended choice.

Sigil (free, Mac/Windows/Linux)

Sigil is a free, open-source ePub editor. Unlike Vellum and Atticus, which generate ePub from your manuscript, Sigil is an editor for ePub files you already have — you import an ePub and edit its HTML, CSS, and metadata directly. Sigil is the tool for authors who need to make precise manual corrections to an ePub file, check or edit its table of contents, or clean up conversion artifacts. It is not a good starting point for ebook production but is invaluable for troubleshooting and repair.

Draft2Digital's Formatter

Draft2Digital provides a free ePub generator — upload your Word document and D2D creates an ePub for distribution across its partner network. The output is clean and functional for standard prose. It lacks the design polish of Vellum or Atticus but is fully functional for most fiction and nonfiction. Authors who do not want to invest in a dedicated formatting tool and are distributing through D2D can use D2D's formatter as a no-cost option.

Calibre (free)

Calibre is a free ebook management and conversion tool. It can convert between ebook formats and has been used by authors for ebook production. However, Calibre's conversion output is notoriously inconsistent — it often produces ePub files with structural or CSS problems that cause rejection on platforms like Apple Books. Calibre is better used as an ePub management and format-checking tool than as a primary production tool for publication-quality ePub.

Front Matter Requirements

Front matter is the content that appears before the main text of your book. What you include and how it's structured affects both reader experience and platform metadata alignment.

The Functional Table of Contents

In ePub formatting, there are two types of table of contents: the visual TOC (an HTML page in the book that readers see) and the functional/navigation TOC (the NCX or nav.xhtml file that reading devices and apps use to navigate). Both are required in a properly structured ePub. The functional TOC must correctly link to every chapter — broken TOC links are one of the most common causes of Apple Books rejection and reader complaints on all platforms.

Formatting tools like Vellum and Atticus generate both automatically. If you are producing ePub manually or editing an ePub in Sigil, verify that both TOC types are present and all links resolve correctly.

Back Matter Requirements

Back matter — the content after the final chapter — is critical marketing infrastructure for indie authors. What you include and how you structure it directly affects reader conversion to newsletter subscribers, series read-through, and review generation.

Back Matter Links on Wide Platforms

If you include links in your back matter — to your next book, your newsletter sign-up, or your website — use universal links that work regardless of which platform the reader purchased on. An Amazon-specific book link in your Kobo ebook's back matter sends Kobo readers to a store they may not use. Use Books2Read universal links (books2read.com) for book links and direct URLs for your website and reader magnet delivery.

Cover Requirements by Platform

Practically: design your cover at a minimum of 2,400 × 3,600 pixels (6:9 ratio, portrait) and it will meet or exceed the minimum requirements on every platform. Export as high-quality JPEG for upload. Keep the file under 10 MB by adjusting JPEG compression if needed.

Validating Your ePub Before Upload

ePub validation checks your file against the ePub specification and catches the structural errors that cause platform rejections. Validating before you submit is the single most effective way to avoid the rejection-and-resubmission cycle, particularly for Apple Books.

ePub Validator (IDPF / W3C)

The free online ePub validator at validator.idpf.org (also accessible through the W3C) checks your ePub file against the ePub 2 and ePub 3 specifications. Upload your .epub file and the validator returns a list of errors and warnings. Errors must be fixed before submitting to Apple Books. Warnings are less critical but should be addressed if possible.

Kindle Previewer (Amazon)

Amazon's free Kindle Previewer application (available for Mac and Windows from KDP's resources page) converts your ePub or .docx to Kindle format and previews how it will appear on Kindle Paperwhite, Kindle Fire, and the Kindle app on phones and tablets. Always preview your KDP submission in Kindle Previewer before publishing. Conversion artifacts — broken chapter headers, missing spaces, image scaling issues — that are invisible in your source file are visible in the Kindle preview.

Sigil's ePub Check

Sigil includes a built-in ePubCheck function (Tools > Validate ePub with ePubCheck) that runs the same validation as the online IDPF validator. If you use Sigil for any ePub editing, run ePubCheck after any change before re-uploading.

Common Formatting Problems and Fixes

Formatting Checklist Before Every Upload

• ePub file validated with IDPF validator or Sigil's ePubCheck — no errors

• Kindle Previewer review complete for any KDP submission — formatting looks correct

• Both visual TOC (HTML page) and functional TOC (NCX/nav) present and all links resolve

• Cover image meets the target platform's minimum dimension and ratio requirements

• All fonts embedded if custom fonts are used

• Back matter includes series read-next links using universal links or author website links

• No competitor retail links in back matter for KDP submissions

• Copyright page includes year, author name, publisher name, and ISBN if applicable

• File size under platform maximum (typically 10–50 MB depending on platform)

Ebook formatting done right is invisible to the reader — they simply experience a book that reads beautifully on their device. Ebook formatting done wrong is immediately visible: broken chapters, missing content, ugly text rendering, dead links in back matter. The investment in a quality formatting tool and a pre-upload validation habit pays off in better reader experiences, fewer complaints, and books that meet Apple's quality review on the first submission rather than the third.

-Randall Wood