Inksky
Amazon’s ebook formats have evolved through three generations, and the naming is genuinely confusing if you’re new to the ecosystem. Here’s what each format means in practice and which one you should use when converting or sideloading books.
MOBI: the original (legacy)
MOBI (Mobipocket) was Amazon’s first ebook format, purchased in 2005. It’s the oldest of the three and, in 2026, should be treated as legacy. MOBI files are limited: no custom fonts, no enhanced typesetting, no hyphenation, and no support for newer Kindle features. They work on every Kindle ever made, which is their only remaining advantage.
Amazon announced MOBI deprecation for Send-to-Kindle in 2022, and while MOBI files sent through the service still process, they lack modern formatting features. If you’re converting books today, skip MOBI entirely. The only reason to produce a MOBI file is if you’re sending a book to someone with a Kindle from 2010 or earlier.
AZW3 (KF8): the workhorse
AZW3 — also called Kindle Format 8 (KF8) — arrived in 2011 alongside the original Kindle Fire. It’s still the most practical format for USB sideloading in 2026. AZW3 supports embedded fonts, CSS3 styling, SVG images, and fixed-layout children’s books. It doesn’t support enhanced typesetting (that’s KFX-only), but it renders well on every Kindle from the Kindle Touch onward.
If you’re converting EPUBs in Calibre and sideloading via USB, AZW3 is the right choice. It preserves formatting better than MOBI, handles non-Latin scripts (CJK, Arabic) properly, and gives you control over fonts and margins. The trade-off: AZW3 books sideloaded via USB won’t sync reading position across devices — that requires KFX or Amazon’s cloud delivery.
KFX: the modern format
KFX is Amazon’s current-generation format, introduced in 2015 and now the default for all books purchased directly from the Kindle Store. It’s the only format that supports enhanced typesetting — Amazon’s proprietary rendering engine that adds hyphenation, improved kerning, better word spacing, and ligatures. Enhanced typesetting makes a visible difference in text quality, especially on higher-resolution screens like the Paperwhite’s 300 ppi display.
KFX also enables:
- Faster page turns (pre-rendered text)
- Bookerly and Amazon Ember as default fonts
- Page flip (navigating without losing your place)
- Whispersync of reading position across devices
- Word Wise and X-Ray integration
The catch: KFX is a compiled, DRM-wrapped format. Converting to KFX requires the Calibre KFX Output plugin, and the conversion process is more complex than AZW3. You also can’t easily edit KFX files after conversion.
Which format should you use?
| Scenario | Recommended Format | Why |
|---|---|---|
| Buying from Kindle Store | KFX | Automatic, full feature support |
| Sideloading via USB | AZW3 | Best balance of compatibility and formatting |
| Send-to-Kindle (cloud) | EPUB | Amazon converts to KFX automatically |
| Archiving your library | EPUB (not Kindle format) | Open standard, future-proof |
| Maximum text quality on device | KFX | Enhanced typesetting |
| Simple, no-fuss conversion | AZW3 | Works, looks good, no extra plugins |
What about KF7?
You might see references to KF7. It’s essentially MOBI with a different name — Amazon’s original format before AZW3/KF8. Any device that reads MOBI reads KF7. Ignore the distinction; it’s historical trivia at this point.
The practical workflow
For most people, the best approach is simple: send EPUBs to Kindle using Send-to-Kindle. Amazon converts them to KFX automatically, and you get cloud sync and enhanced typesetting without touching a USB cable. For USB sideloading — which you’d use for privacy, large files, or batch transfers — convert to AZW3 in Calibre. For the absolute best text rendering and you’re willing to install the KFX Output plugin, convert to KFX. But honestly, the difference between a well-formatted AZW3 and a KFX file is subtle enough that most readers won’t notice.