What Is an EPUB File?

EPUB (Electronic Publication) is the most widely used ebook file format. It's an open standard maintained by the W3C that stores reflowable text, images, and metadata in a single compressed file.

How EPUB Works

An EPUB file is actually a ZIP archive containing:

• XHTML files — the book's text content (chapters)
• CSS stylesheets — formatting and typography
• Images — cover art, illustrations, diagrams
• OPF file — metadata (title, author, language) and reading order
• Navigation document — table of contents

Because EPUB uses web standards (HTML + CSS), the text reflows to fit any screen size — from phones to desktop monitors.

EPUB 2 vs EPUB 3

EPUB 2 (2007) supports basic text and images. EPUB 3 (2011, current) adds audio, video, JavaScript interactivity, vertical text, and improved accessibility. Most modern ebooks use EPUB 3, but readers support both versions. You can check which version your file uses with our EPUB validator.

What Devices Read EPUB?

Nearly everything except Kindle (which requires conversion): Apple Books, Google Play Books, Kobo, Nook, and hundreds of third-party apps. See our full guide on how to open EPUB files.

Working with EPUB Files

Epublys offers free tools to work with EPUBs directly in your browser:

• Merge multiple EPUBs into one
• Split by chapters
• Compress to reduce file size
• Convert to PDF
• Edit metadata (title, author, cover)
• Validate for errors

Related Guides

• How to open EPUB files on any device
• EPUB vs PDF
• EPUB vs MOBI
• How to send EPUB to Kindle