AZW3 (also called Kindle Format 8 or KF8) was Amazon's 2011 answer to EPUB 3 — a major upgrade to the original MOBI/AZW format that added proper HTML5 and CSS3 support, SVG images, and embedded fonts. It's the format Kindle devices and apps use when rendering an ebook with advanced formatting.
The key distinction from its predecessor MOBI: AZW3 is based on a container (like EPUB) rather than a flat binary file. It supports multiple HTML5 files, proper CSS stylesheets, embedded fonts, and SVG. Visually, an AZW3 ebook can look nearly identical to a well-formatted EPUB — the differences are in ecosystem support, not rendering quality.
Confusingly, Amazon's Kindle ecosystem uses both file extensions interchangeably in some contexts. A file you download from Amazon might be .azw3, .azw, or handled internally as KFX. For sideloading purposes, AZW3 (.azw3) is the format that preserves the most formatting fidelity on Kindle devices.
EPUB and AZW3: Technical Overview
EPUB 3: The Web-Based Open Standard
EPUB 3 is a ZIP archive containing HTML5, CSS3, SVG, images, fonts, and OPF metadata. It's essentially a self-contained mini-website. The W3C/IDPF maintain the specification, ensuring it evolves with web standards. EPUB 3 supports the full CSS3 cascade, web fonts (WOFF/WOFF2), JavaScript interactivity (in EPUB 3.2+), MathML for equations, audio and video embedding, and both reflowable and fixed-layout profiles.
Platform support is universal except for Kindle: Apple Books, Kobo, Google Play Books, Nook, Readium, Adobe Digital Editions, and library systems (OverDrive, Libby) all read EPUB natively. When you submit an ebook to Amazon KDP, you submit EPUB and Amazon converts it to their internal format.
AZW3 (KF8): Amazon's Enhanced Kindle Format
AZW3 uses a container similar to MOBI's binary Palm Database format but allows multiple HTML files and proper CSS stylesheets. It was created specifically to match EPUB 3's capabilities on Kindle hardware, enabling publishers to create visually rich ebooks that display correctly on Kindle devices with proportional fonts, drop caps, CSS-styled chapter headings, and embedded images.
AZW3 supports CSS3 (though with Kindle-specific quirks), embedded TrueType and OpenType fonts, SVG illustrations, and the KF8 navigation format. What it doesn't support: JavaScript execution in most Kindle contexts, the full CSS3 spec (certain properties behave differently), and direct playback outside the Kindle ecosystem.
Amazon's internal KFX format (2015+) has superseded AZW3 for new Kindle purchases, adding Bookerly font support, enhanced typography, and better image handling. However, AZW3 remains the format you create for sideloading — most conversion tools output AZW3, not KFX (KFX requires Amazon's proprietary KindleGen or KDP for creation).
Technical Comparison: EPUB vs AZW3
| Feature | EPUB 3 | AZW3 (KF8) |
|---|---|---|
| Standard type | Open (W3C/IDPF) | Proprietary (Amazon) |
| Container format | ZIP + HTML/CSS/OPF | Palm DB container + HTML/CSS |
| CSS support | Full CSS3 | CSS3 subset (Kindle quirks) |
| Fonts | WOFF/WOFF2 embedding | TrueType/OpenType embedding |
| SVG support | Full | Full (in Kindle viewers) |
| JavaScript | Yes (EPUB 3.2+) | Limited (most Kindles disable) |
| Fixed layout | EPUB 3 FXL spec | KF8 Fixed Layout (similar) |
| Metadata | Dublin Core + OPF | MOBI record + OPF-like |
| DRM | Adobe DRM / LCP | Amazon KFX DRM (for purchased) |
| Open in non-Amazon apps | Any EPUB reader | Kindle apps only |
| Created by open tools | Any EPUB editor/Calibre | Calibre, KindleGen (deprecated), KDP |
Platform and Device Support
| Platform | EPUB | AZW3 |
|---|---|---|
| Kindle Paperwhite / Oasis / Scribe | Not directly (KDP accepts EPUB) | Full (via USB sideload) |
| Kindle Fire / Tablet | Not directly | Full |
| Kindle iOS / Android app | Not directly | Full |
| Apple Books (iOS / macOS) | Native | Not supported |
| Kobo e-readers | Native | Not supported |
| Google Play Books | Native | Not supported |
| Barnes & Noble Nook | Native | Not supported |
| Calibre (desktop) | Full read/edit | Full read/convert |
| OverDrive / Libby | Native | Not supported |
| Readium | Native | Not supported |
File Size Comparison
For equivalent ebook content, EPUB and AZW3 file sizes are generally similar. AZW3 may be slightly larger in some cases because its binary container has overhead that EPUB's ZIP compression handles more efficiently.
| Content | EPUB size | AZW3 size |
|---|---|---|
| Text-heavy novel (300 pages) | 200–400 KB | 250–500 KB |
| Nonfiction with illustrations | 5–20 MB | 5–22 MB |
| Children's picture book | 10–30 MB | 10–35 MB |
When to Use EPUB vs AZW3
Use EPUB When...
- Publishing on any non-Amazon platform — Apple Books, Kobo, Google Play Books, Nook, libraries: EPUB is required
- Creating a master archive copy — EPUB is the origin format; convert to AZW3 from it, not the other way around
- Publishing on KDP — Amazon accepts EPUB for KDP submissions and converts internally to their format
- Making your ebook widely accessible — A DRM-free EPUB works in Calibre, every reading app, and every platform. AZW3 only works in Amazon's ecosystem
Use AZW3 When...
- Sideloading to a Kindle device via USB — AZW3 sideloads with better formatting fidelity than original MOBI; it's the best option for USB transfer to Kindle
- You have existing AZW3 content to manage — Calibre reads AZW3 for library management and conversion to other formats
- You need Kindle-specific typography optimization — AZW3 formatting is tuned for Kindle's rendering engine, which can produce slightly better results than a generic EPUB conversion on Kindle hardware
Convert EPUB to AZW3 (or AZW3 to EPUB) with ChangeThisFile
ChangeThisFile supports EPUB ↔ AZW3 conversion via /epub-to-azw3 and /azw3-to-epub. Conversions use Calibre's ebook-convert, the best open-source tool for ebook format conversion.
curl -X POST https://changethisfile.com/v1/convert \
-H "Authorization: Bearer YOUR_API_KEY" \
-F "file=@ebook.epub" \
-F "target=azw3" \
-o ebook.azw3690 routes supported. Free for 1,000 conversions/month. No SDK required.