2026-07-11 · all guides

EPUB Formatting Rules: What Actually Matters in a Reflowable Ebook

The mental model: an EPUB is a website in a box

An EPUB file is a zip archive of HTML files, one or a few per chapter, plus CSS, images, and metadata. The reading system, a Kindle, Apple Books, Kobo, or a phone app, lays that HTML out live for the current screen size and the settings the reader has chosen. This is what reflowable means: there is no fixed page. Rotate the phone, bump the font size, and every line break and page break in the book moves.

Everything that confuses print-minded authors about ebooks follows from this. You cannot control where pages end, you cannot rely on your chosen font being used, and anything you position absolutely will drift on some device. The craft of EPUB formatting is doing less: clean structure, relative sizing, and no fighting the reading system.

Rule one: no fixed sizes, no page numbers

Sizes in an EPUB should be relative: ems and percentages, not points and pixels. A heading set to 1.5em stays fifty percent larger than body text whatever size the reader chooses; a heading fixed at 18 points may end up smaller than the body when a low-vision reader scales the text up. The same goes for margins and indents, and images should be constrained to a maximum of one hundred percent of screen width so they never overflow a small phone.

Page numbers simply do not exist in a reflowable book. Do not print them in headers or footers, do not reference them in the text, and strip them from the table of contents, which must be hyperlinks instead. Phrases like as we saw on page 84 must become as we saw in chapter 5. Kindle shows readers locations and estimated progress instead, and that is not something you format; it is computed.

Rule two: structure with real headings, style with restraint

Reading systems and conversion tools understand your book through its HTML structure. Chapter titles must be real heading elements, h1 or h2, not enlarged bold paragraphs, because the EPUB navigation document, the built-in contents menu, is generated from the heading structure, and KDP flags ebooks whose navigation is broken. Each chapter should start a new HTML file, which also guarantees a page break before it on every device.

Styling beyond that should be minimal and defensive. Readers can and do override fonts, so font choice is a suggestion; embedding a font is permitted but adds file size, and Kindle delivery fees on the 70 percent royalty plan are charged per megabyte, so large embedded assets literally cost you money per sale. First-line indents or space between paragraphs, pick one convention, both is a classic conversion artifact. Avoid multi-column layouts, text wrapped around images, and anything that assumes a page shape.

Validation and the KDP pipeline

Before uploading, validate. EPUBCheck is the standard validator and the same class of checks retailers run at ingestion; an EPUB that fails validation may be rejected or may render unpredictably. Then preview on real conditions: the KDP online previewer, plus at minimum one phone-size screen and one larger font size, because those two variations surface ninety percent of reflow problems, orphaned images, overflowing tables, and headings styled as body text.

KDP accepts EPUB directly for Kindle ebooks and converts it internally, so a clean EPUB is the whole job; you do not need a separate Kindle format anymore. If you are producing both editions, remember they are different artifacts: the print PDF is fixed and designed, the EPUB is fluid and structured. ebookdone generates both from the same manuscript, with the EPUB built as structured chapters with linked navigation, so the reflowable edition is not an afterthought export.

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