Kindle KFX file format

.kfx · archive (e-book) · introduced 2015 by Amazon

What is a Kindle KFX file?

KFX is Amazon's third-generation Kindle book format, used since 2015 for typographically-rich Kindle titles. The container holds many compressed sub-files for chapters, fonts, and layout descriptors. KFX always ships with Amazon DRM; tools to convert it to ePub exist but are restricted to your own legally-purchased books.

Most readers reach this page after double-clicking a .kfx file and finding their operating system unsure what to do with it. The good news: Kindle KFX is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a Kindle KFX file

Opening a .kfx file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .kfx file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .kfx file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.kfx    # works for almost any format

The files appear in the current directory.

How to create a Kindle KFX file

To pack files into a .kfx archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write Kindle KFX.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .kfx files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Rich typography
Resolution-independent images

Weaknesses and limitations

Always DRM-locked
Closed format
Kindle ecosystem only

Typical use cases

Kindle e-book distribution
Modern Kindle layout

Technical details

Kindle KFX uses the Custom (LZW/zlib + DRM) algorithm and is identified on disk by the magic byte sequence 5A 4F 30. The standard MIME type is application/vnd.amazon.ebook-kfx. The format is a proprietary specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is Kindle KFX safe to open?

Archive files are containers — they are only as safe as what they contain. A .kfx from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a Kindle KFX?

Some archivers can wrap Kindle KFX files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does Kindle KFX compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.