What is the KFX file format?
.kfx files are Kindle Format 10 (KF10) e-books - Amazon's most advanced proprietary e-book format. Introduced in 2015 with Kindle Firmware 5.6 for 7th-generation Kindle devices, KFX superseded the older AZW3 format as the default for new Kindle content and is the format delivered to current Kindle devices and apps.
KFX is built on Amazon Ion binary, a hierarchical data format that is a binary-encoded superset of JSON. A complete book is typically distributed as a KFX-ZIP - a standard ZIP archive containing multiple Ion entity files representing different portions of the book: text content, metadata, fonts, images, and annotation positions. A manifest entity enumerates all others. On-device delivery stores fragment files in a shared directory instead.
The format introduces significantly enhanced typesetting compared to older Kindle formats: proper kerning, ligatures, soft-hyphenation, the custom Bookerly typeface, and HTML5/CSS3-style layout rendering. Embedded images may use JPEG XR (.jxr) for better compression alongside standard JPEG and PNG. DRM is voucher-based and account-tied via Kindle Serial Number - a stronger scheme than the XOR-based protection used in AZW/MOBI.
Amazon generates KFX server-side; third-party creation is not officially supported. Calibre with the KFX Input/Output plugins by jhowell is the main tool for reading and writing DRM-free KFX outside the Amazon ecosystem.
Security & safety
RISK: LOWKFX is e-book data; no execution risk. The friction is DRM: account-locked books refuse to open outside official Kindle apps, and conversion is blocked - a rights restriction, not malware. Pirated KFX download sites may bundle adware in the download wrapper. Treat DRM-free KFX files (personal content) as ordinary documents.
Format details
in a nutshell- Kofax Group 4 Bitmap - Proprietary fax/document scanning bitmap format by Kofax; completely unrelated to Kindle; very rarely encountered today.
Programs that open KFX files
Windows2 apps
Kindle for PC
Free Read KFX books delivered to your Amazon account; the only reliable option for DRM-protected books.
Calibre (with KFX Input plugin)
Open-source Install Calibre, then add the KFX Input plugin (Preferences → Plugins → Get new plugins → search KFX). Open DRM-free .kfx files, read in the built-in viewer, and convert to EPUB/PDF/AZW3. 
macOS2 apps
Calibre (with KFX Input plugin)
Open-source Install KFX Input plugin; open and convert DRM-free KFX files. 
Linux1 app
Calibre (with KFX Input plugin)
Open-source Primary option on Linux; install KFX Input plugin from Preferences → Plugins. 
Android1 app
Technical details
deep spec| Container | Amazon Ion binary (binary-encoded JSON superset); typically distributed as KFX-ZIP (standard ZIP archive) |
| Ion binary magic | 0xE0 0x01 0x00 0xEA (Ion Binary Version Marker) at the start of each Ion entity file |
| KFX-ZIP magic | Standard ZIP PK header (0x50 0x4B 0x03 0x04) when distributed as a KFX-ZIP bundle |
| Byte order | Big-endian (Amazon Ion binary format) |
| Compression | DEFLATE compression within the KFX-ZIP wrapper; Ion entity files stored uncompressed inside the ZIP |
| DRM | Voucher-based, account-tied via Kindle Serial Number + account token; stronger than AZW3 XOR-based scheme |
| Typesetting | Kerning, ligatures, soft-hyphens, Bookerly custom typeface, HTML5/CSS3 layout rendering |
| Image formats | JPEG, PNG, and JPEG XR (.jxr) for higher-compression images in illustrated or fixed-layout books |
| Typical size | 1 MB - 50 MB (text novels); up to 200 MB for fixed-layout or heavily illustrated titles |
| Entity structure | Manifest entity + content/metadata/font/image/annotation entities; on-device delivery as a fragment folder |
| Device support | Kindle 7th generation and later; Kindle for PC, Mac, Android, and iOS apps |
| Third-party tools | Calibre with KFX Input/Output plugins (jhowell) reads and writes DRM-free KFX files |
| Content generation | KFX produced server-side by Amazon; no official third-party KFX authoring path exists |
| Released | 2015 (Kindle Firmware 5.6.x, 7th-gen Kindle) |
| Latest version | KFX-ZIP variant (distributed as standard ZIP containing multiple Ion fragments) |
| Specification | wiki.mobileread.com |
KFX conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about KFX files.
