Amazon's Kindle format explained: how it works, how it compares to MOBI, KFX and EPUB, and how to get your books into it.
AZW3 is the ebook format Amazon introduced in 2011 with the Kindle Fire and has used for Kindle books ever since. You will also see it called KF8 (Kindle Format 8), which is its technical name. If you have ever opened the documents folder of a Kindle, the files ending in .azw3 are your books in this format.
Under the hood, an AZW3 file is very close to an EPUB: the text is HTML, the styling is CSS, and images, fonts and a table of contents are packed together into one file. The difference is the container and the fact that Amazon controls the specification. That closeness is the reason converting between EPUB and AZW3 is fast and produces very faithful results.
MOBI was the original Kindle format, inherited from the Mobipocket reader that Amazon bought in 2005. It was designed for very early e-ink screens and supports only basic formatting: no custom fonts, limited tables, very simple CSS. AZW3 was created to fix exactly those limitations.
| Feature | MOBI | AZW3 |
|---|---|---|
| Introduced | 2000s (Mobipocket) | 2011 (Kindle Format 8) |
| Embedded fonts | No | Yes |
| Tables and complex layout | Very limited | Full support |
| CSS styling | Minimal | Modern CSS |
| Kindle compatibility | Every Kindle ever made | Kindles from 2011 onwards |
| Amazon status | Legacy, no longer accepted for Send to Kindle | Current personal-document format |
In practice: if your Kindle was made in the last decade, AZW3 will look noticeably better than MOBI, especially for books with images, tables, poetry or non-Latin scripts. MOBI is only worth choosing if you own one of the very first Kindle models.
Since around 2015 Amazon also uses KFX (Kindle Format 10) for books sold in its store. KFX adds Amazon's typography engine (Bookerly font, improved hyphenation and justification) and is what you get when you buy a book on a modern Kindle.
You do not need to worry about KFX for your own files. Amazon does not publish the format, no third-party tool creates it, and Kindles read AZW3 perfectly. For any ebook you convert yourself, AZW3 is the best format you can produce and the one Amazon recommends for personal documents.
EPUB is the open standard used by Kobo, Apple Books, Google Play Books and virtually every reader that is not a Kindle. AZW3 is Amazon's equivalent. The two are technically similar enough that a well-made EPUB converts to AZW3 with practically no loss.
The rule of thumb is simple: EPUB everywhere, AZW3 on Kindle. Kindle devices do not open EPUB files directly, so converting is the standard step for anyone who buys or downloads books outside the Amazon store.
Note on DRM: books bought in the Kindle store are protected and cannot be converted to other formats. Conversion only applies to DRM-free files, such as books from Project Gutenberg, Standard Ebooks, Humble Bundle or independent authors who sell without DRM.
All of them from 2011 onwards, which covers almost every Kindle in use today:
Only the Kindle 1, Kindle 2, Kindle DX and Kindle Keyboard (3rd generation) predate AZW3 and need MOBI instead.
The easiest way is online, with no software to install:
Upload your EPUB file to the EPUBKit converter
Click Convert to AZW3 — it takes a few seconds
Download the file or send it straight to your Kindle with one click
No. AZW3 is the successor of MOBI. It is built on the same container but adds full support for modern HTML and CSS, so fonts, tables, images and layout look much better. MOBI works on every Kindle; AZW3 works on Kindles released from 2011 onwards.
Not directly. AZW3 is an Amazon format. On phones and tablets you can use the Kindle app, and on Kobo or other eReaders you need to convert the file to EPUB first (this only works with DRM-free files).
On a Kindle, yes: AZW3 is the format the device is optimized for. Everywhere else EPUB is better because it is an open standard supported by almost every reader. The two formats are technically very similar, which is why conversion between them is fast and reliable.
Upload your EPUB to EPUBKit's free EPUB to AZW3 converter and click Convert. You can then download the file or send it straight to your Kindle email address with the Send to Kindle button.