What Is ePub? — Self-Publishing Glossary

ePub is to ebooks what MP3 is to music: the dominant open standard that works across most platforms and devices.

How ePub Works

An ePub file is essentially a compressed zip file containing:

HTML files — the actual book content, one per chapter
CSS stylesheets — the formatting rules (fonts, spacing, drop caps, headers)
Images — cover, any interior images
Metadata files — title, author, ISBN, language, table of contents structure

When you open an ePub on your Kobo or Apple Books, the reader app interprets this package and renders it in a readable format.

ePub vs. Kindle Format

Amazon built their empire on a proprietary format (originally MOBI, now KFX). This means:

For Amazon only: upload your .docx or ePub to KDP, they convert it. Quality varies.
For Apple Books: requires ePub
For Kobo: requires ePub
For Barnes & Noble: requires ePub
For Draft2Digital (wide distribution): requires ePub

If you want to sell anywhere besides Amazon, you need a properly formatted ePub.

What Makes an ePub “Good”

Not all ePub files are equal. A good ePub:

Has clean, well-structured HTML (not the messy output from Word)
Embeds fonts correctly so your typography renders as intended
Has a valid table of contents that works on all readers
Renders correctly on Kindle, Kobo, Apple Books, and older e-ink devices
Passes ePub validation (no errors in the file structure)

Vellum-produced ePub files meet all of these criteria. Word-exported ePub files often don’t.

How to Get a Professional ePub

Option 1: Buy Vellum ($199, Mac only) — format it yourself with the best tool available.

Option 2: Buy Atticus ($147, Mac/Windows) — good output, cross-platform.

Option 3: Use a formatting service — MekarPad formats your manuscript with Vellum and delivers clean .epub and .pdf files starting at $75/book.

Option 4: Reedsy Book Editor (free, web-based) — decent output for simple manuscripts, limited design options.

Common Mistakes

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Submitting a Word .docx file to retailers and calling it an ebook — .docx is not an ePub and won't be accepted by most platforms.

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Using Word's 'Save as ePub' feature — the output is technically valid but produces messy HTML that renders poorly on many devices.

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Assuming ePub and MOBI are the same — Amazon uses KFX/MOBI format, while all other major retailers use ePub. You need both if publishing wide.

Frequently Asked Questions

What's the difference between ePub and MOBI?

ePub is the open standard format used by Apple Books, Kobo, Barnes & Noble, and most non-Amazon retailers. MOBI (and its successor KFX) is Amazon's proprietary format for Kindle devices. If you publish on Amazon only (KDP Select), you need a Kindle-compatible file. If you publish wide, you need an ePub for all non-Amazon platforms. Vellum exports both.

How do I create an ePub file?

The best way is to use dedicated book formatting software: Vellum (Mac, $199), Atticus (Mac/Windows, $147), or the free Reedsy Book Editor. You can also hire a formatting service (like MekarPad, from $75) to produce a professional ePub from your Word manuscript.

Does Amazon accept ePub files?

Yes. Amazon KDP accepts ePub for upload and converts it to their Kindle format automatically. However, the conversion quality varies. Professional formatting services and tools like Vellum optimize the ePub specifically for clean Amazon conversion.

What is ePub3 vs ePub2?

ePub3 is the current standard (released 2011, updated since) with better support for multimedia, complex layouts, and CSS styling. Most modern e-readers support ePub3. ePub2 is the older standard and still widely compatible. Vellum exports ePub3. Most formatting services use ePub3 by default.

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