All Things About Kindle KFX Format to Help You Manage Kindle KFX Books

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Although the Kindle KFX format was released in 2015, it significantly impacted Amazon Kindle book readers in early 2017 when the Kindle for PC/MAC automatically updated to version 1.19. Since then, nearly all Kindle books downloaded via the Kindle desktop application have been in KFX format.

Subsequently, users discovered that Kindle books in this format could only be opened with Kindle devices or Kindle-related applications. So, what exactly is the Kindle KFX format? This is what I aim to share with you. I will address the concerns you may have regarding Kindle KFX. As the old saying goes, "Know thyself, ever-victorious." Let's uncover the mysterious veil of the Kindle KFX format.

1. What is a Kindle KFX file?

Kindle KFX format is the successor to the azw3 format and released with the Kindle Paperwhite 3. This new format was featured a new enhanced typesetting engine and new Bookerly font. The Amazon Kindle books with enhanced typesetting enabled feature can be downloaded in Kindle KFX format.

To further dig, the Amazon uses a new image format called JXR which is higher compressed than JPG.

For downloaded content of the Kindle KFX format, you will not see a single book under the My Kindle Content folder, but a subfolder included the whole content of the book.

For the Kindle KFX eBook downloaded via Kindle for PC/MAC, it contains the content like the following picture.

Kindle KFX download via Kindle for pc/mac

For the Kindle KFX downloaded via Kindle device, it will be a little bit different.

The encrypted eBook content is stored directly in Document folder of my Kindle voyage with the title as: Telling Lies (A Sam Mason Mystery Book 1)_B06XQ655PC.kfx.

At the Document folder, you can also find the .sdr folder with the same name as the book title: Telling Lies (A Sam Mason Mystery Book 1)_B06XQ655PC.sdr. All userful contents of this book is stored at a subfolder named assets. Below are the contents in assets folder.

Kindle content device

If you miss any file, the book cannot be considered as an integrated book.

2. What is Kindle kfx-zip file?

kfx zip in calibre

When I upload my newly downloaded Kindle books via kindle for pc to calibre, I found them display as kfx-zip file. There's no way for me to open such files. what's the difference between kfx and kfx-zip?

The Kindle KFX-ZIP file is not the actual ebook downloaded to your computer via Kindle for PC or Mac. Only Calibre users will encounter KFX-ZIP files when they fail to remove Kindle KFX DRM with Calibre DeDRM. Additionally, they must have the Calibre input and output plugins properly installed. We can consider the presence of KFX-ZIP files as an indication of the failure to remove Kindle KFX DRM using Calibre.

Now we can distinguish between KFX and KFX-ZIP, right? The KFX file is the Kindle book downloaded via Kindle for PC/Mac, while the KFX-ZIP is the file generated by Calibre when DRM removal fails.

3. Why Can't I send to Kindle a kfx? What Devices and Apps Support Kindle KFX Format?

Although Kindle KFX has been released for many years, not all Kindle devices or Kindle reader apps support this format because it is use special rendering techique. The following table shows the Kindle kfx format supported devices and apps.

  Kindle devices with firmware<5.8 Kindle devices with firmware=5.8 Kindle for PC v1.19 and above Kindle for PC v1.21 Kindle for iOS v6.03 Kindle for Android v7.2 Kindle Previewer v2.94
kfx × v(with extension .kfx) v v


v(with extension .kfx) ×

v: KFX format can be read on the device or with the app.
×: KFX format cannot be read on the device or with the app.

Why Can't I send to Kindle a kfx? Send-to-kindle delivering KFX format to Kindle Devices

The "Send to Kindle" tool converts non-Kindle formats into Kindle's native format (KFX) and synchronize them with your library. However, it cannot process formats that are already proprietary Kindle formats, such as KFX. That's why you cannot send to kindle a kfx file.

Instead, utilize "Send to Kindle" with your non-DRM EPUB files. This feature will seamlessly convert EPUB files to KFX and transmit them to your library via cloud sync, while preserving metadata, typically completing the process in just a minute.It is worth noting that only Kindle ereaders that support KFX format are getting personal documents delivered in KFX now (at least the Kindle Paperwhite 3 and newer).

4. How to Avoid Kindle KFX Format? I don't want to download kindle books in kindle kfx format.

Although the KFX format has many new features, but it seems it is not welcome by many readers as it has much more limitations than before. Here I list three ways to avoid Kindle KFX format.

Method 1: Degrade your Kindle for PC/MAC to version 1.17 or older version

From the Kindle for PC/MAC version 1.19, the download books are mostly in Kindle KFX format. So we just have to keep our Kindle for PC/MAC at an old version and the download kindle books will not be in Kindle KFX format. If your Kindle for PC/MAC has already been updated to version 1.19 or even the lastest version, you have to follow the below steps to downgrade it.

Step 1: Deregister you Kindle for PC/MAC

Launch your Kindle for PC/MAC, click on "Tools"--"Options"--"General"--"Registration"--"Deregister". Now it will pop up a new window as below.

Kindle for pc deregister

Then check "I understand……this device" and then click on "Deregister". Now your Kindle account has been deregistered and all downloaded books have been deleted. To ensure all books have been deleted, you can go to My Kindle Content folder at your computer and delete all file at this folder.

Step 2: Uninstall the Kindle for PC/MAC

Right click on the Kindle icon at your desktop, select "open file location", then find the "uninstall.exe" and double click it to uninstall your Kindle.

Step 3: Install the Kindle for PC/MAC version 1.17

Download the old version of Kindle for PC/MAC. Since these install packages are not available at Amazon now, I have collect them for you.

Download the Kindle for PC version1.17.

Download the Kindle for Mac version 1.17.

When you have installed the old version of Kindle at your computer, just run it. Then click “Tools”--“General”--uncheck “Automatically install updates when they are available without asking me”--Click on “Save”. Your Kindle for PC/MAC will stop update automatically.

From now on the books you download will be in mobi, azw or other normal Kindle formats, but not Kindle KFX.

Method 2: Stop the renderer-test function at Kindle for PC/MAC

This method will prevent Kindle for PC/MAC version from version 1.19 to 1.24 from downloading the Kindle books as KFX format, but it can not be applied to Kindle for PC version 1.25 and newer.

From the Kindle for PC/MAC version 1.19, the book with enhanced typesetting function can be downloaded as Kindle KFX format. Do you know the real reason? That’s because the Kindle desktop application use a special rendering technique which called renderer-test. If we stop this module from working, the books downloaded via the new Kindle for PC/MAC will not be in Kindle KFX format. Before the following operation, please close your Kindle for PC/MAC at first.

If you are using Windows system, just open a command window and then copy and paste the following command to your command window and click “Enter” to disable the renderer-test program.

ren %localappdata%\Amazon\Kindle\application\renderer-test.exe renderer-test.xxx

Or you can just right click the Kindle icon at your desktop, then choose “open file location”, then find renderer-test.exe at the open folder, and change the program to renderer-test.xxx.

If you are using the Mac system, open your terminal, copy and paste the following code to the terminal window. Then click on “Enter”.

chmod -x /Applications/Kindle.app/Contents/MacOS/renderer-test

Some of you may come across the chmod command fails with a permission error like the below picture.

renderer test excute

When you enter ls –l /Applications/Kindle.app/Contents/Macos/, you will see that you do not have the root privilege to stop executing the renderer-test program. So you have to elevate the current user privileges to root user privileges. Copy and paste the command Sudo chmod -x /Applications/Kindle.app/Contents/MacOS/renderer-test then click “Enter” and fill in your current user’s passwords and click “Enter”. Then enter ls –l /Applications/Kindle.app/Contents/Macos/, you will see that renderer-test program stops working.

renderer test 2

When the command has been executed, the book you previously downloaded in KFX format will no longer open. You have to remove them all from “My Kindle Content” and re-download the books again. Good news is that the downloaded books will not be in Kindle KFX format this time.

Method 3: Download Kindle eBooks directly from the Amazon website

If you download the Kindle eBooks from the Amazon website, you possibly will not get the Kindle KFX format eBook, but azw3 format. You can follow the detailed guide to download the Kindle eBooks as AZW3 format.

5. How to Remove Kindle KFX DRM

It is universal known that Kindle kfx has been encrypted with Kindle kfx DRM, so if we want to back up our Kindle books, or read it on other reader apps or eReader, we have to remove Kindle KFX DRM at first. Here meet our Epubor Ultimate—the first tool on the market can decrypt and convert Kindle KFX to epub/mobi/pdf/azw3. The decrypted and converted file will keep the original quality.

Step 1. Download and Install Epubor Ultimate.


Epubor Ultimate works on both Windows and Mac system smoothly.

Step 2. Run Epubor Ultimate to remove Kindle kfx DRM.

When you run Epubor Ultimate, the books you downloaded via Kindle for PC/MAC, or Kindle device will be side loading at the left column on the interface. Just drag the books from the left column and drop them to the right column. All books will be decrypted automatically.

remove kindle KFX drm

You can find the decrypted books by clicking the "Decrypted" at the end of each book or clicking the folder icon on right bottom.

Video tutorial:

6. How to Open and convert Kindle kfx file with Calibre

Recently, I received many emails about the problem on reading DRM-free Kindle KFX book with Calibre. Many readers complain that even they have removed DRM from the Kindle KFX eBooks, but they still cannot be open or read these books with Calibre. As we know that the Calibre can support most of the eBooks formats on the market, such as azw, mobi, epub, azw3, pdf, cbr, and so on, but Kindle KFX is not in its list. From part 3, we know that even the Kindle for PC/MAC cannot open the Kindle KFX eBooks when the renderer-test module stops working. Since Calibre does not have this Amazon exclusive rendering technique, consequently it cannot open the Kindle KFX eBooks. If you want to open kfx with Calibre, you have to convert KFX to epub/mobi/pdf/azw3 using the Epubor Ultimate.

Some Calibre users may wonder if they can convert the Kindle kfx file with Calibre? Yes, but you need to install Dedrm plugin to Calibre first and then convert kindle kfx to PDF/epub with Calibre.

convert drm free kfx file

Now you can read Kindle KFX with Calibre. These are all things about Kindle KFX format I collected for you guys. If you still have concerns about the Kindle kfx, please leave them at the comments.

Frequently Asked Questions
Q: If I fail to remove Kindle kfx DRM, what should I do?

A: You can try to download Kindle books as AZW3 format and remove the DRM or read the article Kindle KFX DRM Remove Failed and Solution to fix your problem.

Q: Can I remove Kindle kfx DRM from books downloaded via Android device?

A: It might not be possible to remove kfx DRM from books obtained from Kindle for android apps. But you can download kindle books via Android emulator and then remove drm with the Epubor Kindle Converter.

Q: Can I use Calibre to convert Kindle KFX format eBooks now? How to?

A: With the latest De-DRM plugin installed, you can convert KFX to pdf/epub/mobi/txt with Calibre. Please read How to convert Kindle kfx to epub/pdf/mobi with calibre to learn how to convert kfx to pdf with Calibre converter.


has been a passionate member of Epubor since 2017. Her mission is to enhance your ebook reading experience by sharing insightful tips and tricks. Join her on a journey towards reading excellence today!

01/31/2018 20:59:58
Thanks for all this detailed - if to my slow old mind a little confusing. I will save it and read it again when the .kfx impinges on my system.
02/1/2018 08:36:31
We are glad to know you like this article. We will keep sharing everything we know about the ebook and audiobooks!
04/1/2018 23:30:09
I deleted renderer-test.exe from the user/local location and Calibre now converts azw books without a problem.
04/2/2018 10:14:45
Yes, that's a solution.
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