Kindle readers don’t take “any ebook.” Over the years Amazon moved from MOBI to AZW3 and now to KFX, while Send to Kindle added EPUB support. This page untangles the formats so you convert to the right one instead of a file your Kindle rejects. > BookConv processes your file on our servers and automatically deletes it within 1 hour. Files are encrypted in transit, and no account or software install is required.
The Kindle format landscape
MOBI: the old standard. Amazon retired MOBI side-loading, so new uploads in MOBI are no longer the recommended path.
AZW3: Amazon’s modern ebook format with better typography and features; widely supported by Kindle devices.
KFX: Amazon’s current proprietary format with advanced layout; created by Amazon’s own pipeline, not easily produced by third-party converters.
EPUB: the open standard. Send to Kindle now accepts EPUB and converts it for your device.
Kindle format comparison table
| Format | Sideload support | Modern features | Third-party readable | BookConv can produce |
|---|---|---|---|---|
| AZW3 | Yes (all Kindles) | Yes (CSS, fonts, tables) | No (Kindle only) | Yes |
| KFX | No (Amazon only) | Yes (advanced layout) | No | No |
| MOBI | Yes (legacy USB) | No | Partial | Yes (to EPUB or AZW3) |
| EPUB | Via Send to Kindle | Yes (open standard) | Yes (most readers) | Source format |
For sideloading your own file, AZW3 is the most compatible format BookConv can produce. KFX is Amazon-only, so convert to AZW3 instead.
What Is AZW3?
AZW3 is Amazon is KF8 format, introduced in 2011 to replace the original AZW — a thin wrapper around the old Mobipocket engine. Under the hood it is HTML and CSS packaged much like EPUB, but sealed inside the company-owned own container, which is why only Kindle hardware and apps read it.
What that buys you over the older MOBI/AZW1 lineage:
- Embedded fonts and real CSS, so typography survives the trip
- Tables, drop caps, and fixed-layout pages for illustrated titles
- Better spacing and margin control than Mobipocket ever allowed
It is not an open format, so Kobo, Nook, and most third-party readers ignore it. Think of AZW3 as the best format inside Amazon is walled garden, and EPUB as the one that travels everywhere else.
Moving in or out is simple: EPUB to AZW3 for a modern Kindle, or AZW to MOBI if a legacy device cannot read AZW3.
What to convert to (practical rule)
For sideloading a file yourself: convert to AZW3 — BookConv can produce it and Kindles read it well.
For Send to Kindle: upload EPUB or AZW3 and let Amazon convert.
Avoid: converting new books to MOBI, and trying to generate KFX yourself.
Common Kindle conversion paths
EPUB → AZW3 for sideloading: EPUB to AZW3.
AZW3 → PDF if you need a fixed-layout printout: AZW3 to PDF.
MOBI → EPUB/AZW3 to modernize an old library: Mobi to EPUB.
AZW → MOBI only if a legacy Kindle cannot read AZW3: AZW to MOBI.
Why not just use MOBI?
MOBI is legacy. Amazon's Send to Kindle no longer prioritizes it, and newer features only exist in AZW3/KFX. Converting new content to MOBI risks a file your device handles poorly.
If you already own MOBI files, convert them forward to AZW3 rather than keeping the old format.
One important detail: MOBI files from the pre-2011 era often contain proprietary DRM and encoding that modern tools struggle with. Even if they open, typography and layout may look broken on newer Kindles. This is another reason to migrate your library to AZW3 or EPUB as your first priority.
What is KFX and why you cannot create it
KFX is Amazon's newest proprietary format, introduced around 2017 as the successor to AZW3 for Store purchases. It uses advanced typesetting features including better typography, image handling, and layout control that go beyond what AZW3 offers.
The critical limitation: KFX is generated exclusively by Amazon's own conversion pipeline. When you purchase a book from the Kindle Store, Amazon creates the KFX version server-side. Third-party converters like BookConv cannot produce true KFX files.
What this means for you:
- You cannot sideload KFX to your Kindle via USB
- If a third-party tool claims to produce KFX, it is likely producing an AZW3 file with a renamed extension
- For sideloading, AZW3 remains your best option — it has nearly all the visual features of KFX
KFX is not a format you need to worry about as a user. It is Amazon's internal delivery format for purchased books. Your job is to ensure your sideloaded files are AZW3 or that you use Send to Kindle for EPUB uploads.
Kindle device compatibility matrix
Not all Kindles support all formats equally. Here is the definitive compatibility table:
| Kindle Model | AZW3 | KFX | MOBI | EPUB (Send to Kindle) |
|---|---|---|---|---|
| Kindle (1st–4th gen) | ❌ | ❌ | ✅ | ❌ |
| Kindle Touch | ❌ | ❌ | ✅ | ❌ |
| Kindle Paperwhite 1st gen | ❌ | ❌ | ✅ | ❌ |
| Kindle Paperwhite 2nd–4th gen | ✅ | ✅ | ✅ | ✅ |
| Kindle Voyage | ✅ | ✅ | ✅ | ✅ |
| Kindle Oasis (all gens) | ✅ | ✅ | ✅ | ✅ |
| Kindle Scribe | ✅ | ✅ | ✅ | ✅ |
| Kindle Fire tablets | ✅ | ✅ | ✅ | ✅ |
| Kindle Kids (2019+) | ✅ | ✅ | ✅ | ✅ |
Key takeaway: Every Kindle released after 2015 supports AZW3. If you have a Kindle from before 2015, stick with MOBI or consider upgrading your device. For the most complete model-by-model breakdown, see our can-Kindle-read-AZW3 guide.
Rendering engine comparison: how formats actually work
Understanding the rendering engine behind each format helps you predict what will and will not work:
Mobipocket Engine (MOBI/AZW): The original Kindle engine. Supports basic text flow and simple images. No CSS, no embedded fonts, limited layout control. This is why MOBI files look plain and cannot carry rich typography.
KF8 Engine (AZW3): Amazon's second-generation engine. Full CSS support, embedded fonts, table support, drop caps, and better image handling. AZW3 files render much more closely to their intended design. This is the engine used by most Kindles released after 2011.
KFX Engine: Amazon's current proprietary engine. Extends KF8 with improved image compression, advanced typography features, and better handling of complex layouts. KFX is optimized for Amazon's own pipeline and cannot be generated externally.
Why this matters for conversion: When you convert EPUB to AZW3, you are moving from an open standard (EPUB, which uses web browser rendering engines) to Amazon's KF8 engine. Most content converts faithfully. Complex layouts may lose some styling, but the core text and images survive. For heavily styled books like textbooks or cookbooks, always preview the converted file before relying on it.
Future trends: what comes after KFX?
Amazon has not announced a public successor to KFX, but several trends are shaping the future of Kindle reading:
Send to Kindle becoming the primary path. Amazon increasingly pushes users toward Send to Kindle rather than sideloading. This means EPUB is the preferred upload format, and Amazon handles the KFX conversion on their servers. For most users, this is the simplest and most reliable workflow.
AZW3 remaining relevant for sideloading. As long as users want to sideload files via USB, AZW3 will remain the format of choice. It is well-supported, produces good results, and does not require Amazon's pipeline.
DRM restrictions tightening. Amazon has been phasing out MOBI side-loading support, which suggests they may eventually deprecate other sideloading formats as well. If this happens, Send to Kindle (EPUB) will become the only viable path for personal file transfer.
What to do now: Convert your existing MOBI library to AZW3 or EPUB. Use Send to Kindle for new acquisitions. Keep AZW3 as your backup sideloading format. This strategy covers all current and foreseeable Kindle behaviors.
Can Kindle Fire Read MOBI?
Kindle Fire tablets run the Kindle app on Fire OS and read the same formats as other Kindles: MOBI, AZW3, KFX, and EPUB through Send to Kindle. So yes, a Fire can open MOBI files.
One caveat: older MOBI files built with non-standard tools sometimes fail to open on a Fire and show the familiar error that the file does not open. If a MOBI will not open, convert it to AZW3 — BookConv handles that — or resend it through Send to Kindle, which re-encodes the file. For a model-by-model look at which formats each Kindle reads, see our can-Kindle-read-AZW3 guide.
Related guides for your specific situation
If you are not sure which format fits your device or use case, these deeper guides help:
- Can my Kindle read AZW3? — model-by-model compatibility table. /blog/can-kindle-read-azw3
- Your ebook will not open on Kindle? — five common causes and fixes. /blog/why-ebook-wont-open-kindle
- EPUB to AZW3 step-by-step — convert without losing formatting. /guide/epub-to-azw3-for-kindle
- Moving from Kindle to Kobo? — why EPUB is the right target. /blog/mobi-to-kobo
- Upgrading AZW3 to MOBI for old devices — compatibility guide. /blog/azw3-to-mobi
- FB2 and Lit format reference — non-Amazon library formats explained. /blog/fb2-vs-epub and /blog/lit-format-conversion-and
- Troubleshooting non-Kindle readers — Kobo, Apple Books, and Android tips. /blog/kobo-to-epub and /blog/read-epub-on-any-device
Start converting now
Ready to convert? Head to the EPUB to AZW3 converter for a modern Kindle, or MOBI to EPUB if you want to escape the Amazon ecosystem entirely. Both run in your browser with no install. For the format-level detail behind this page, the AZW3 format reference sets out its capabilities and limits.