AZW3 (also known as KF8 or Kindle Format 8) is Amazon's modern ebook format, replacing the aging MOBI standard in 2011. It brought Kindle's ebook capabilities in line with EPUB 3, supporting modern HTML5, CSS3, embedded fonts, and complex layouts. Understanding AZW3 is essential for anyone publishing to Kindle or managing a modern Kindle library.
What Is AZW3?
AZW3 was introduced alongside the Kindle Fire tablet in 2011 to address the fundamental limitations of the older MOBI/AZW format. While MOBI used a simplified HTML subset with no stylesheet support, AZW3 is built on:
- HTML5 for content markup
- CSS3 for typography and layout
- OpenType/TrueType embedded fonts (subset embedding like EPUB)
- SVG for scalable graphics
- Fixed-layout support (essential for children's books, comics, textbooks)
- Media overlays (audio synchronization for read-aloud books)
Structurally, AZW3 uses Amazon's proprietary container format wrapping the same technology stack as EPUB 3. This is why Calibre can convert between AZW3 and EPUB with high fidelity — they're built on the same HTML/CSS foundations.
AZW3 vs MOBI vs EPUB
| Feature | AZW3 | MOBI | EPUB 3 |
|---|---|---|---|
| CSS support | ✅ Full CSS3 | ❌ None | ✅ Full CSS3 |
| Embedded fonts | ✅ Yes | ❌ No | ✅ Yes |
| Fixed layout | ✅ Yes | ❌ No | ✅ Yes |
| HTML5 | ✅ Yes | ❌ Subset only | ✅ Yes |
| DRM | Optional (Amazon DRM) | Optional | Optional |
| Open standard | ❌ Proprietary | ❌ Proprietary | ✅ W3C standard |
| Kindle support | ✅ Full | ✅ Legacy | Via Kindle app only |
| Non-Kindle readers | ❌ Limited | ❌ Limited | ✅ Universal |
Where AZW3 Is Used
AZW3 is the delivery format for all modern Kindle Store purchases:
- Enhanced ebooks with custom fonts and complex layouts
- Children's books with fixed-layout pages
- Comics and manga (Kindle Comixology integration)
- Textbooks with complex tables, math equations, and multi-column layouts
- Kindle Unlimited content
When you buy a Kindle book today, you're almost certainly getting AZW3 (or KFX for premium content) rather than the legacy MOBI format.
Converting AZW3 Files
AZW3 to EPUB (for Kobo, Apple Books, Google Play Books):
ebook-convert book.azw3 book.epub
Calibre produces excellent EPUB output from AZW3 since both share the HTML5/CSS3 foundation. The conversion preserves fonts, styles, and layout.
AZW3 to PDF:
ebook-convert book.azw3 book.pdf
For reflowable books, Calibre produces a good PDF. For fixed-layout AZW3 (comics, children's books), PDF conversion is more reliable because page dimensions are already defined.
AZW3 to MOBI (backward compatibility):
ebook-convert book.azw3 book.mobi
This downgrades the format — you'll lose CSS styling, embedded fonts, and any fixed-layout features. Only do this if targeting very old Kindle devices.
EPUB to AZW3 (for self-publishers):
ebook-convert book.epub book.azw3
Amazon's Kindle Previewer tool also converts EPUB to AZW3 and shows exactly how the book will look on Kindle devices.
DRM on AZW3
Amazon-purchased AZW3 files have DRM applied. The technical DRM format is called "Topaz" for older files and "KFX" for premium content. DRM-free AZW3 files (from Smashwords, Draft2Digital, direct author sales) convert without restrictions using Calibre.
Creating AZW3 Files (Self-Publishers)
If you're a Kindle Direct Publishing (KDP) author:
- Write in EPUB: create your book in EPUB format using tools like Vellum (Mac), Sigil, or Reedsy
- Upload EPUB to KDP: Amazon converts to AZW3 automatically
- Use Kindle Previewer: download Amazon's Kindle Previewer desktop tool to see exactly how your book renders across Kindle generations before publishing
- Fixed-layout guide: for children's books or comics, use KDP's Fixed Layout guide with specific CSS viewport declarations
Kindle Format Roadmap
AZW3 remains the primary format, but Amazon continues developing:
- KFX: used for enhanced Kindle books with features beyond AZW3 (Whispersync for Voice, print replica)
- Print Replica: PDF-in-Kindle-wrapper for textbooks
- Kindle Vella: serialized fiction platform using a web-first format
For most ebook publishing and conversion needs, AZW3 ↔ EPUB conversion via Calibre covers everything.
Related conversions
Frequent conversions across the catalogue: