When managing Kindle ebooks in Calibre, you may often see two plugins mentioned together:
- KFX Input plugin: Convert KFX or kfx-zip file to epub, pdf or other format.
- KFX Output plugin: Convert any format to kindle kfx format.
Both plugins are related to the Kindle KFX format, but they serve completely different purposes. Understanding how they work will help you manage Kindle ebooks more effectively, especially when converting formats or removing DRM.
This article explains what these plugins do, when you need them, and how to deal with kfx with Calibre Kfx input and kfx output plugin.
What Is the KFX Format? (And Why AZW3 is Aging)
For years, AZW3 (KF8) was the best kindle format for side-loading and transfering. But Amazon’s newer KFX (Kindle Format 10) is the current king. KFX isn't just a file extension; it’s an engine that enables Enhanced Typesetting.
- Hyphenation and Justification: No more awkward gaps between words.
- Page Flip: The ability to "zoom out" and skim multiple pages at once.
- Dynamic Leading: Better spacing that adjusts to your font size.
Because KFX is a proprietary format, Calibre cannot read it natively, which is why special plugins are needed.
KFX Input
What Is the KFX Input Plugin?
The KFX Input plugin is about getting your books out of the Kindle ecosystem. Amazon now delivers almost all new books in KFX format. If you try to drag a book from your Kindle into Calibre without this plugin, Calibre won't even know what it's looking at. It might appear as a .kfx-zip or a folder of encrypted fragments.
What the KFX Input Plugin Does?
Once installed, the plugin allows Calibre to:
- Import .kfx files
- Import KFX-ZIP packages
- Read Kindle books downloaded by Kindle apps
- Extract metadata and covers
- Convert KFX ebooks to other formats
It essentially acts as a translator, allowing Calibre to understand Kindle’s KFX format.
1 Import Kindle Books into Calibre.
When you download a book using a Kindle application, the file may appear as: .kfx or
.kfx-zip.
Without the plugin, Calibre may show errors or fail to recognize the format.
After installing the KFX Input plugin, Calibre can successfully import and process the file.
2 Convert Kindle Books to EPUB
Many users want to read Kindle books on other devices such as Kobo or PocketBook.
The common workflow is: Kindle book >> Calibre + KFX Input + NoDRM Plugin>> DRM removal (if applicable)>>Convert to EPUB.
3 Manage Kindle Books in Calibre Library
With the plugin installed, you can: view metadata, edit book details, convert formats, organize your ebook library.
Does kfx input plugin remove drm directly?
The KFX Input plugin allows Calibre to read the format, but it does not automatically remove DRM.
If your book is "locked," you will need the latest DeDRM tools (often found on GitHub via the NoDRM fork).
In 2025-2026, Amazon increased encryption on KFX. The most reliable way to use KFX Input and Nodrm together for DRM removing purpose. The latest NODRM Plugin will generate kfx-zip file directly. These files are already drm-free but in the kfx-zip format and KFX input is required for converting this kfx-zip to epub or pdf format.
How to Install Calibre KFX Input Plugin?
1 Download the KFX Input plugin from the MobileRead plugin thread.
After downloading, you will get a plugin file like this: KFX Input.zip.
2 Launch Calibre, then go to Preferences and select Plugins. At the bottom of the Plugins window, click Load plugin from file. Next, locate the plugin file you downloaded (usually named KFX Input.zip), select it, and click Open. Calibre will display a security warning stating that installing plugins from third-party sources may be dangerous. Simply click Yes to continue.
3 Restart Calibre after the installation is complete so that the plugin can take effect.
Note: Do not unzip the plugin file before installing it.
KFX Output
What Is the KFX Output Plugin?
The KFX Output plugin does the opposite to KFX Input plugin. Instead of importing Kindle books, it allows Calibre to convert ebooks into the KFX format. This is useful if you want to create high-quality Kindle files from EPUB or other formats.
The KFX Output plugin is for the perfectionist. Its sole job is to take your high-quality EPUBs and convert them into the specialized KFX format that Kindle loves.
What the KFX Output Plugin Does?
The plugin enables conversions from EPUB to KFX, AZW3 to KFX, or MOBI to KFX.
How to install KFX Output Plugin to Calibre?
The most common mistake users make is thinking the plugin works alone. It doesn't. To use KFX Output, you must install Amazon Kindle Previewer 3 on your PC or Mac. The plugin is essentially a "bridge" that sends your book to the Previewer’s engine to be professionally compiled.
1 Install Kindle Previewer 3 from Amazon's website.
2 In Calibre, go to Preferences >> Plugins >> Get new plugins . Search "KFX Output" and install KFX Output plugin.
3 When converting a book, go to the "KFX Output" tab and ensure you’ve added a valid ASIN (Amazon Standard Identification Number) in the metadata. This tricks the Kindle into enabling X-Ray and Goodreads integration.
4 KFX files are complex. Do not use "Send to Kindle" (email). Amazon’s cloud service will often reject or downgrade .kfx files. Use a good old-fashioned USB cable for the full experience.
Some important information of Kfx output plugin:
- 1. The plugin does not generate KFX files by itself. Instead, it uses Kindle Previewer, Amazon’s official publishing tool. Actual conversion pipeline: Ebook>> Calibre converts to EPUB >> Kindle Previewer >> KPF >> KFX.
- 2. The KFX Output plugin requires Kindle Previewer to be installed on the computer. Without it, KFX conversion will fail.
- 3. Certain types of books are harder to convert to KFX, such as Comic books, Manga, Fixed-layout ebooks, Image-heavy books, and Complex textbooks. These formats may fail or produce layout issues after conversion.
- 4. In recent years, Amazon’s Send-to-Kindle service has improved. When users send EPUB files to Kindle, Amazon often converts them automatically into KFX format. Because of this change, you may no longer need to manually create KFX files. However, the plugin is still useful for users who prefer full control over their ebook conversion workflow.
KFX Input vs. KFX Output: At a Glance
| No. | Feature | KFX Output | KFX Input |
|---|---|---|---|
| 1 | Direction | EPUB >> Kindle(KFX) | Kindle >> EPUB / PDF / AZW3 |
| 2 | Primary Goal | Better fonts, Page Flip, X-Ray | Backup, conversion, cross-platform reading |
| 3 | Must-Have | Kindle Previewer 3 installed | Latest DeDRM / NoDRM (for encrypted Kindle books) |
Final Thoughts
The KFX Input and KFX Output plugins significantly expand the capabilities of Calibre when working with Kindle ebooks.
KFX Input plugin allows Calibre to read and convert Kindle KFX files.
KFX Output plugin allows Calibre to generate KFX ebooks for Kindle devices.
For most users, installing KFX Input is essential when managing Kindle books in Calibre.
The KFX Output plugin is mainly useful for advanced users who want to create optimized KFX files for sideloading to Kindle devices.
Iris Yan is a passionate member of the Epubor team, dedicated to enhancing your ebook reading experience. She shares insightful tips, tricks, and practical advice to help you get the most out of your digital library. Join her on the journey to reading excellence today!
