EPUB 3.3 and ONIX 3.0 for indie authors: what KDP actually needs
Two acronyms now decide whether your ebook is accepted, discoverable, and legal to sell in the EU: EPUB 3.3 (the file) and ONIX 3.0 (the metadata about the file). The rules tightened in 2025–2026, and most 'export to ebook' buttons haven't caught up. Here's what changed and what you actually have to ship.
EPUB 3.3 — the book; ONIX 3.0 — the label on the box
An EPUB is the ebook itself: your chapters, structure, and styling in a standardized package that e-readers know how to open. EPUB 3.3 is the current W3C standard, and it bakes in accessibility — semantic structure, navigation, and text that screen readers can actually read.
ONIX is different: it's the metadata record that travels with your book through the supply chain — title, contributors, price, subjects, and, crucially, accessibility features. Retailers and libraries read ONIX to list, sort, and surface your book. ONIX 3.0 is the current version; ONIX 2.1 has been formally retired.
What changed in 2025–2026 (and why your old export may fail)
Two deadlines reshaped the landscape. First, the EU Accessibility Act took effect in June 2025: ebooks sold into the EU must meet accessibility requirements and must declare their accessibility features in metadata. Second, the industry retired ONIX 2.1 — Amazon and others moved to require ONIX 3.0.
The practical consequence: a file exported by older software may be a valid EPUB but still get rejected or down-ranked because its metadata is the wrong version or doesn't declare accessibility. 'It opens in my Kindle app' is no longer the bar.
Your store-ready checklist
Before you upload, a compliant package generally needs:
- A validated EPUB 3.3 — passes EPUBCheck with no errors, with proper headings, a navigation document, and language set.
- Accessibility metadata — Schema.org accessibility features declared (e.g. structural navigation, readable text), so EU listings are compliant by default.
- ONIX 3.0 metadata — current version, with your title, contributors, subjects (BISAC/Thema), and accessibility features.
- A print-ready PDF if you're also doing paperback — correct trim size, bleed, and embedded fonts.
You shouldn't need a developer for this
Hand-editing XML or wrangling EPUBCheck on the command line is not a good use of an author's time. The point of modern publishing tools is to emit a validated EPUB 3.3 + ONIX 3.0 package — accessibility metadata included — from the manuscript you already wrote.
Scribegrove's Publishing Studio does exactly that: validated EPUB 3.3, ONIX 3.0 metadata, and a KDP-ready PDF, EU-Accessibility-Act compliant by default, with step-by-step submission help for KDP, Draft2Digital, and IngramSpark. It never automates against the publisher's portal — you stay in control of the actual upload — and it never takes a cut of your royalties.
Frequently asked
Does Amazon KDP require ONIX 3.0?
The industry retired ONIX 2.1 and moved to ONIX 3.0, so current metadata should be 3.0. Always confirm the exact requirement in KDP's current help pages before you upload, since store requirements change.
Do I need to worry about the EU Accessibility Act if I'm not in the EU?
If you sell into the EU market — which most KDP authors do by default — the requirements apply to those sales. Shipping accessibility metadata by default is the safe path; it doesn't hurt your other markets.
This guide is general information for authors, not legal advice. Platform and store policies change — verify the current terms wherever you publish.