What is EPUB? Everything You Need to Know

EPUB is the world's most widely used open e-book format. Here's what it is, how it works, and why it matters for your reading experience.

EPUB at a Glance

EPUB stands for Electronic Publication. It's an open standard maintained by the W3C (World Wide Web Consortium, the same organization that maintains HTML and CSS standards). The current version is EPUB 3.3, released in 2023.

Unlike PDF, which preserves exact page layouts, EPUB is a reflowable format. This means the text adapts to your screen size and reading preferences — it reflows to fit your device, whether that's a phone, tablet, e-reader, or desktop monitor. You can change font size, font family, and margins, and the text adjusts accordingly.

How EPUB Works Under the Hood

An EPUB file is actually a ZIP archive with a .epub extension. Inside, you'll find:

• mimetype — A plain text file identifying the archive as an EPUB
• META-INF/container.xml — Points to the main content file
• OPF file (Open Packaging Format) — The book's manifest and metadata (title, author, language, etc.)
• XHTML files — The actual book content, one file per chapter
• CSS stylesheets — Formatting and typography rules
• Images — Cover art, illustrations, and other graphics
• NCX/Nav file — The table of contents for navigation

Because EPUB uses standard web technologies (HTML, CSS, and XML), it's inherently flexible and well-suited for reflowable content.

EPUB 2 vs EPUB 3

EPUB 2 (2007) was the original widely-adopted version. It uses XHTML 1.1, a subset of CSS 2, and the NCX format for navigation. Most older e-books use this version, and it's still fully supported by all readers.

EPUB 3 (2011, updated through 2023) is the modern version. Key improvements include:

• HTML5 and CSS3 support for richer layouts and typography
• Embedded audio and video
• JavaScript interactivity (for enhanced e-books)
• Improved accessibility features (ARIA, semantic markup)
• Better support for right-to-left languages and vertical text
• MathML for mathematical notation
• Fixed-layout mode for comics and children's books

Who Uses EPUB?

EPUB is the standard format for virtually every major e-book platform except Amazon's Kindle Store:

• Apple Books — Uses EPUB as its primary format
• Google Play Books — Accepts and delivers EPUB
• Kobo — Primary format for all Kobo devices
• Barnes & Noble Nook — Uses EPUB
• Public libraries (via OverDrive/Libby) — Distribute in EPUB
• Project Gutenberg — Offers free classic books in EPUB
• Amazon Kindle — Now accepts EPUB via Send to Kindle (since 2022)

EPUB and Kindle

Historically, Kindle was the one major e-reader that didn't support EPUB. Amazon used its proprietary MOBI and AZW3 formats, forcing users to convert their EPUB files before reading on Kindle.

That changed in 2022 when Amazon's Send to Kindle service began accepting EPUB files directly. While the Kindle Store still delivers books in Amazon's proprietary KFX format, you can now send any DRM-free EPUB to your Kindle without conversion.

For the best experience, it's recommended to prepare your EPUB files by cleaning up metadata and ensuring proper structure before sending to Kindle.

EPUB vs Other E-book Formats

• EPUB vs PDF: EPUB is reflowable (adapts to screen size); PDF is fixed-layout (preserves exact formatting). EPUB is better for novels and text-heavy books; PDF is better for manuals, forms, and graphically complex documents.
• EPUB vs MOBI: EPUB is an open standard with better features; MOBI is Amazon's deprecated proprietary format. Read our detailed comparison.
• EPUB vs AZW3: AZW3 (KF8) is Amazon's enhanced format based on EPUB 3 concepts but proprietary. It's what the Kindle Store uses internally.

How to Open EPUB Files

You can read EPUB files on virtually any device:

• E-readers: Kobo, Nook, and most non-Kindle devices support EPUB natively
• Kindle: Send via email or the Send to Kindle app
• iOS/macOS: Apple Books (built-in)
• Android: Google Play Books, Moon+ Reader, ReadEra
• Windows: Calibre, Sumatra PDF, Thorium Reader
• Linux: Calibre, Foliate
• Web: Various browser-based readers

DRM and EPUB

Some EPUB files are protected with Digital Rights Management (DRM), which restricts copying and sharing. The most common DRM schemes for EPUB are Adobe DRM and Apple's FairPlay DRM.

DRM-protected EPUBs can only be opened in apps authorized for your account. They cannot be processed by third-party tools like ConvertToKindle. If you encounter a DRM-protected EPUB, you'll need to read it in the app or platform where you purchased it.

Have EPUB files you want to read on Kindle? Use our free converter to prepare them with clean metadata — all processing happens in your browser.