EPUB vs DOCX: which file should you upload, and when it matters

What each format actually is

An EPUB is a zip package of web pages: your chapters as XHTML, a stylesheet, the images, and a manifest that tells reading apps what is inside and in what order. Because it is reflowable, the reader controls the presentation — font, size, margins, light or dark — and the text pours to fit any screen. That is the point of the format, and it is why an ebook has no fixed "page 214". EPUB 3 is the current standard across stores.

A DOCX is a word-processing document: perfect for writing and editing, never sold to readers directly. When a platform "accepts DOCX", it converts your document into its ebook format at upload — reading your heading styles as chapter breaks, your italics and bold as formatting, and your embedded images as the ebook’s images. The conversion is only as good as the document’s structure, which is where the traps live.

What each platform accepts

Ebook upload formats by platform (as documented, 23 August 2026)

Print is a different pipeline everywhere: a fixed PDF interior plus a cover file, not an EPUB.

Read the table backwards and the practical rule appears: a good EPUB is accepted everywhere (Apple takes nothing else), while DOCX coverage is broad but not universal. Authors publishing wide therefore end up with one finished EPUB as the master file sooner or later — the only question is whether a converter makes it or you do.

When DOCX is the right upload

For a text-first book — a novel, a memoir, most narrative non-fiction — uploading the DOCX to a platform that converts is not a compromise; it is the standard route. The conversion succeeds when the document is structurally clean, and "clean" means five habits: chapter titles carry a real heading style (Heading 1), not big bold text; paragraph indents come from paragraph settings, not tabs or spaces; no manual page numbers, headers or footers (the ebook makes its own); scene breaks are a centred character, not a run of blank lines; and images sit inline at the point they belong. Do that, and the converter builds the table of contents and chapter navigation from your styles — the preview each platform shows before you publish is where you confirm it worked.

When only a real EPUB will do

Some books outgrow conversion. Poetry, where line breaks are the art. Footnote- and endnote-heavy non-fiction, where linked notes must behave. Books with tables, sidebars or callout boxes that need styling. Illustrated and photo-led books, which often want EPUB’s fixed-layout variant rather than reflow. Boxed sets assembled from several books. And any book where you want to control the final file byte for byte rather than trust seven different converters. The toolchain is friendly now: export EPUB directly from writing apps or convert with the free Calibre, then finish by hand if needed. Two checks before upload: run the file through the free EPUBCheck validator (stores reject invalid packages), and open it in at least one real reading app — a five-minute skim catches what validators cannot, like a broken chapter order.

The megabyte tax: file size is money on Amazon

One platform prices your file size. Amazon’s 70% royalty rate deducts a delivery fee of £0.10 per megabyte in the UK ($0.15 in the US) on every sale. A tidy text novel is about 1 MB — a 10p deduction nobody notices. A book whose photos went in at full camera resolution can be 20 MB — £2 gone from every £4.99 sale, turning a 70% royalty into a 30% one. The fix costs nothing: resize images to the size a screen needs (1,600px on the long edge is plenty for full-page art, far less for spot images) and compress them before they enter the manuscript. No other major store charges delivery fees, which is one more reason image-heavy books lean on Apple, Kobo, Google — or a flat-share platform like Ebokio, where the fee structure is the same ~2.9% + 25p card fee whatever the file weighs.

The 60-second decision Text-first book, platforms that convert → upload the clean DOCX. Anything with designed elements, or going to Apple direct, or that you want byte-identical everywhere → make one validated EPUB and upload it everywhere. Either way, check the on-platform preview before publishing.