---
title: "What Is an EPUB File? The Ebook Format"
description: "What the EPUB ebook format is, what is inside an EPUB file, how reflowable and fixed layouts work, and how KDP treats EPUB uploads."
url: "https://www.superebook.ai/glossary/epub"
updated: "2026-10-20"
---

# EPUB

**Definition.** EPUB is the open file format for ebooks, maintained by the W3C. An EPUB packages a book's HTML, CSS, images and metadata into one ZIP-based file that reading apps lay out on the fly, so text can reflow to fit any screen. KDP accepts EPUB uploads that meet its Kindle Publishing Guidelines and converts them for Kindle.

Sources checked October 20, 2026.

## How it works

The current version is EPUB 3.3, a W3C Recommendation whose latest edition is dated January 2026. Inside the container, a package document lists the book's metadata, every file it uses (the manifest) and the order the chapters are read in (the spine). Chapters are XHTML content documents styled with CSS. By default an EPUB is reflowable, laid out by the reading app for each screen, but metadata can declare a fixed layout with set page dimensions for books that need page-precise design. KDP also accepts Word files and Kindle Create's KPF format, and recommends checking any EPUB in Kindle Previewer before upload.

## An example

Rename a copy of an EPUB to end in .zip and open it: you will find a mimetype file, a META-INF folder that points to the package document, and the chapters as XHTML files. A chapter that is in the manifest but missing from the spine is still inside the file, yet a reader turning pages from the start never reaches it.

## Common mistakes

Treating a PDF as an ebook: a PDF keeps fixed pages and doesn't reflow on a phone. Uploading an EPUB to KDP without opening it in Kindle Previewer first. And carrying print habits into a reflowable book: KDP's reflowable guidelines ask for a single-column layout and no CSS positioning, so text-box layouts copied from print fall apart.

## Questions

**Can I upload an EPUB to KDP?** Yes. KDP supports EPUB files that meet the specifications in its Kindle Publishing Guidelines and converts them for Kindle devices and apps. It recommends checking the file in Kindle Previewer before you upload.

**Should I sell an EPUB or a PDF from my own site?** EPUB reads better on phones and e-readers because it reflows; PDF keeps a designed page and prints well. Many sellers include both, with the EPUB for reading and the PDF for printing.

## Sources

- [W3C: EPUB 3.3 (Recommendation)](https://www.w3.org/TR/epub-33/): EPUB 3.3's status as a W3C Recommendation, the OCF ZIP container, the package document, manifest and spine.
- [W3C: EPUB 3 Overview](https://www.w3.org/TR/epub-overview-33/): EPUB as a distribution format for web content in a single-file container; reflowable and fixed layouts.
- [KDP: What file formats are supported for eBook manuscripts?](https://kdp.amazon.com/en_US/help/topic/G200634390): KDP's supported ebook formats: DOC/DOCX, KPF and EPUB meeting the Kindle Publishing Guidelines, checked in Kindle Previewer.
- [KDP: Creating Reflowable Books](https://kdp.amazon.com/en_US/help/topic/GPNJPYK298J8TRRV): Single-column layout and no CSS position property in reflowable books.

## Related terms

- [Reflowable vs fixed layout](https://www.superebook.ai/glossary/reflowable-vs-fixed-layout.md)
- [KDP (Kindle Direct Publishing)](https://www.superebook.ai/glossary/kdp.md)
- [ISBN](https://www.superebook.ai/glossary/isbn.md)

## Read next

- [How to format an ebook for Kindle](https://www.superebook.ai/guides/how-to-format-an-ebook-for-kindle.md)
- [PDF, EPUB or DOCX: which file to send](https://www.superebook.ai/blog/pdf-epub-docx.md)
- [How to write an ebook with AI](https://www.superebook.ai/guides/how-to-write-an-ebook-with-ai.md)
- [All glossary terms](https://www.superebook.ai/glossary.md)