What Is an EPUB File?
EPUB is the standard file format for ebooks, used by nearly every major retailer and reading app — Apple Books, Kobo, Google Play Books, and (since 2025) Amazon KDP as well. The name comes from “electronic publication.”
The Core Idea: Reflowable Text
The defining feature of EPUB is that it’s reflowable — the text automatically adjusts to fit whatever screen it’s displayed on, and to whatever font size or style the reader chooses. Whether someone’s reading on a phone, a tablet, or a dedicated ereader, the text reflows to fit, rather than staying fixed like a printed page. This is the opposite of PDF, which is a fixed layout that looks identical everywhere.
What’s Inside an EPUB File
An EPUB isn’t a single document — it’s a packaged file (technically a zipped folder) containing:
- Your book’s text, broken into HTML files
- A stylesheet controlling fonts, spacing, and formatting
- Embedded images and fonts
- Metadata (title, author, description) and navigation information for the table of contents
EPUB 2 vs. EPUB 3
Two versions exist. EPUB 2 is older and more limited. EPUB 3 is the current standard, built on modern HTML5 and CSS3, with support for better accessibility, embedded multimedia, and fixed layouts for visually complex books like comics or children’s picture books. See our full guide to EPUB 3 for the details.
| EPUB | MOBI | ||
|---|---|---|---|
| Status | Current standard | Retired by Amazon (2025) | Used for print, not ebooks |
| Latout | Reflowable | Reflowable | Fixed |
| Used by | Nearly all retailers | Legacy Kindle only | Print-on-demand interiors |
If you have an existing MOBI file, see our guide to editing MOBI files. If you need a PDF for a print edition, see what a PDF file is and when you need one.
How to Open and Read an EPUB File
If you want to check how your own EPUB looks before publishing, or just read an EPUB file you already have, see our guide to reading EPUB files on your laptop or PC.
Get a Properly Formatted EPUB File
A hand-formatted EPUB avoids the broken navigation, styling errors, and validation issues that auto-converted files commonly run into.