Amazon’s Kindle e-readers use a proprietary file format that has changed over time. Since the first Kindle launched in 2007, the company has introduced several versions of its format, each offering new features. Knowing the differences between AZW, AZW3, KFX, and other formats helps you choose the right files for your device and avoid compatibility issues. This article covers the main Kindle eReader format, how they compare, and how to get books onto your Kindle.
What Is the Kindle Ereader Format?
The Kindle File Format is a proprietary e-book file format created by Amazon. It was initially released in 2007 with the filename extension .azw. The format is based on the older Mobipocket (MOBI) format, which Amazon acquired. Over the years, Amazon released several updates to improve layout control, typography, and overall reading experience. Today, Kindle devices support multiple formats, but the native formats remain those developed by Amazon.
The Main Kindle File Formats
AZW – The Original Format
AZW is the first Kindle format, introduced with the original Kindle in 2007. It is based directly on the Mobipocket (MOBI) standard. AZW files have limited design flexibility; they do not support modern HTML5 and CSS3 features. While older Kindle devices and apps can still open AZW files, Amazon has moved toward more advanced formats. AZW is still used for some older content, but it is not the recommended choice for new books.
AZW3 (KF8) – The Modern Standard
Version 8 of the Kindle File Format, known as KF8, was introduced to support HTML5 and CSS3 features. It uses the .azw3 extension. AZW3 allows custom fonts, complex layouts, embedded fonts, and more advanced styling. It is the primary Kindle eBook format for most modern books. When you send an EPUB file to your Kindle through the Send-to-Kindle service, Amazon converts it to AZW3. This format is reflowable, meaning the text adapts to different screen sizes, making it ideal for reading on various Kindle models.
KFX – The Enhanced Format
Version 10 (KFX) was released in August 2015. It introduced an enhanced typesetting and layout engine with support for hyphens, kerning, and ligatures. KFX is optimized for newer Kindle devices and provides a more polished reading experience. The typography adjustments make text easier on the eyes, especially for books with complex formatting. Not all older Kindle devices fully support KFX; it works best on devices released after 2015. KFX is now the latest format from Amazon, but AZW3 remains widely used.
Legacy Formats: MOBI and Its Deprecation
MOBI (Mobipocket) is the format that AZW was based on. For years, authors and publishers could upload MOBI files directly to Amazon’s Kindle Direct Publishing. However, Amazon stopped accepting MOBI uploads for new reflowable books in August 2022. MOBI is now considered a legacy format. If you have older MOBI files, most Kindle devices can still read them, but Amazon no longer supports them for new content. The company now recommends EPUB as the upload format for publishers, which it then converts to its own formats.
Other Supported File Types on Kindle
Kindle devices can display several non-proprietary file types, though they may not support reflowable text for all of them. Supported formats include:
Plain Text (TXT) – basic text with no formatting, reflowable
PDF – fixed layout, does not reflow on small screens
HTML – web pages
RTF – rich text format
DOC/DOCX – Microsoft Word documents
JPEG, GIF, PNG, BMP – image files (for display as pictures)
PDF files maintain their original page layout, which can be challenging to read on smaller Kindle screens because the text does not adjust to fit the display. For the best reading experience, reflowable formats like AZW3 or KFX are preferred.
EPUB and Kindle: How They Work Together
EPUB is a universal open e-book format used across many devices and apps. For a long time, Kindle devices did not natively support EPUB. That changed when Amazon updated its Send-to-Kindle service. Now, the service accepts .epub files and converts them to Amazon’s KF8 (.azw3) format. This means you can upload an EPUB file to your Kindle library, and Amazon will handle the conversion. EPUB is now the recommended format for publishers uploading new books to Amazon, replacing MOBI.
Comparison of Kindle Formats
Format | Extension | Key Features | Current Status |
|---|---|---|---|
AZW | .azw | Original format, based on MOBI, limited design | Legacy but still supported |
AZW3 (KF8) | .azw3 | HTML5/CSS3 support, custom fonts, reflowable | Primary modern format |
KFX | .kfx | Enhanced typesetting, hyphenation, kerning, ligatures | Latest format for newer devices |
MOBI | .mobi | Older format, no longer accepted for new uploads | Deprecated for new content |
EPUB | .epub | Open standard, accepted by Send-to-Kindle for conversion | Preferred upload format |
Fixed layout, no reflow | Supported but not ideal for small screens |
How to Get Books onto Your Kindle
There are several ways to add books to your Kindle device or app. The most common method is purchasing from the Kindle Store, which delivers books in the appropriate format automatically. For books you own from other sources, you can use the Send-to-Kindle service. This service accepts EPUB, PDF, DOCX, and other file types, converts them, and delivers them to your Kindle. Another powerful tool is Calibre, a free and open-source application that can convert many file formats to Kindle-supported formats. Calibre supports conversions to AZW3 and MOBI, though for MOBI you should note Amazon’s deprecation. Using Calibre, you can also manage your library and transfer files via USB.
Which Kindle Format Should You Use?
For most readers, AZW3 (KF8) is the best choice. It offers reflowable text, supports modern design features, and works on nearly all Kindle devices. If you have a newer Kindle model, KFX may provide a slightly better reading experience with enhanced typography. When downloading books from third-party sites, choose AZW3 if available. If you are converting your own files, use Calibre to output AZW3. Avoid PDF for long reading sessions because it does not reflow. EPUB files are fine as long as you send them through Send-to-Kindle, which converts them to AZW3 automatically.
Amazon commands approximately 72% of the e-reader market, so understanding Kindle formats is valuable for anyone who reads digitally. As the ecosystem continues to evolve, Amazon has shifted toward EPUB as the universal input format while relying on its own formats for the best experience on its devices.
Frequently Asked Questions
What is the best Kindle eReader format to use?
The best format for most users is AZW3 (KF8) because it supports HTML5 and CSS3 features, custom fonts, and reflowable text. It works on nearly all Kindle devices. For newer devices, KFX offers enhanced typography with hyphenation, kerning, and ligatures, but it may not be supported on older models.
Can I read EPUB books on my Kindle?
Yes, but not directly. Amazon’s Send-to-Kindle service accepts .epub files and converts them to the KF8 (AZW3) format before delivering them to your device. You can email EPUB files to your Kindle email address or use the Kindle app to upload them. EPUB is now the recommended format for publishers uploading new books to Amazon.
Is the MOBI format still supported by Kindle?
MOBI is a legacy format that Amazon no longer accepts for new reflowable books as of August 2022. Existing MOBI files can still be read on most Kindle devices and apps. However, Amazon recommends using EPUB for uploads. If you have MOBI files, you can convert them to AZW3 using Calibre.
How do I convert a file to Kindle format?
You can use Calibre, a free and open-source tool, to convert many file formats to Kindle-supported formats such as AZW3 or MOBI. Alternatively, you can use Amazon’s Send-to-Kindle service, which converts EPUB, PDF, DOCX, and other files automatically when you send them to your Kindle.
Does PDF work well on Kindle e-readers?
PDF files can be displayed on Kindles, but they are not reflowable. The fixed layout means text does not adjust to different screen sizes, which can make reading uncomfortable on smaller screens. For a better reading experience, use a reflowable format like AZW3 or KFX instead of PDF.
