EPUB
Also known as: .epub, EPUB 3, e-book format, Calibre, IDPF
EPUB is the standard file format for e-books, used by Kobo, Apple Books, Barnes & Noble, and public library e-book systems.
An EPUB file is essentially a website (HTML, CSS, images) compressed into a .epub container, with metadata and a table of contents, designed to reflow text for any screen size.
EPUB, explained at three levels
At home
When you download a library e-book to your Kobo or iPad, it's usually an EPUB file. Unlike a PDF (which has a fixed layout), EPUB text reflows to fit any screen size and font preference. Make the text bigger, and the pages rearrange automatically.
At work
EPUB is the publishing industry's standard for e-books. Publishers produce EPUB files from their book files; distributors (Apple Books, Kobo, library platforms) sell or lend them. Understanding the format is relevant for anyone working in publishing, content management, or documentation delivery.
At home
An EPUB file is a ZIP archive containing: OPF metadata file (title, author, ISBN, chapter list), NCX/nav.xhtml (table of contents navigation), XHTML content files (one per chapter), CSS stylesheets, and images. The OPF file's spine defines reading order. EPUB 3 supports audio, video, MathML, and JavaScript for interactive or fixed-layout publications.
At work
EPUB accessibility requires: semantic HTML structure (headings, lists, landmarks), alt text for images, logical reading order, and metadata in the EPUB package. EPUB 3 Accessibility 1.1 aligns with WCAG 2.1. DRM (Digital Rights Management) is often applied using Adobe ADEPT or Readium LCP, which encrypts content keys and ties them to a user account.
At home
EPUB's content documents use XHTML (strict XML-valid HTML) or HTML5 in EPUB 3. CSS support varies widely between e-reader implementations: a common frustration for EPUB developers. Pandoc converts between EPUB, DOCX, Markdown, PDF, and many other formats. Calibre is the most-used open-source tool for EPUB management and format conversion.
At work
Fixed-layout EPUB (FXL) maintains precise visual layout like a PDF, used for illustrated children's books, magazines, and cookbooks. EPUB CFI (Canonical Fragment Identifier) provides a standardized way to reference specific locations within an EPUB for bookmarks, annotations, and highlights: important for read-position sync across devices.
Real-world examples
At home
Jerome borrows e-books from his public library via the Libby app. The books download as EPUB files with DRM; they're automatically returned after the loan period expires because the DRM enforces the lending period.
At work
Nadia's technical documentation team exports their Sphinx documentation as EPUB for offline reading by field engineers. They use Calibre's command-line tools to post-process the EPUB, fixing styles and embedding custom fonts for consistent rendering across reader apps.
Common confusions
- EPUB and PDF are equivalent for e-books — PDF is fixed-layout: the page looks the same regardless of screen size. EPUB is reflowable: text and layout adapt to the reader's screen, font size, and preferences. EPUB is usually better for prose; PDF is better for content where layout is critical (technical diagrams, forms).
- Kindle reads EPUB files — Kindle devices use Amazon's own formats (AZW3 and KFX). Since 2022, Send to Kindle accepts EPUB files, but it converts them to Amazon's format first, and Kindle Store books are sold in that format, not as EPUB.
For analysts
For BAs involved in content publishing or documentation, EPUB is relevant for delivering reports and documentation in a format that's readable on e-readers and tablets without PDF's fixed-layout constraints.
History & origin
EPUB was created by the International Digital Publishing Forum (IDPF) and published as a standard in 2007, replacing the older Open eBook format. EPUB 3 (2011) added multimedia support, JavaScript, and improved accessibility. In 2017, the IDPF merged with the W3C, which now stewards the format. Amazon's Kindle uses a proprietary format (MOBI/KFX), creating a split in the e-book ecosystem, though since 2022 Amazon's Send to Kindle service accepts EPUB files and converts them to its own format.