What File Format Does Kindle Use? EPUB, KFX & AZW3 (2026)

TL;DR

Kindle devices natively read AZW3 and KFX files, but authors never create those formats directly. If you’re publishing on KDP, upload an EPUB file. Amazon converts it behind the scenes into whatever format the reader’s device needs. MOBI is fully deprecated and no longer accepted anywhere in Amazon’s ecosystem.

The Short Answer (and Why It’s Confusing)

Kindle uses proprietary formats called AZW3 and KFX to display ebooks on its devices and apps. But here’s the part that trips people up: authors don’t upload AZW3 or KFX to Kindle Direct Publishing. They upload EPUB (the recommended choice), and Amazon’s servers handle the conversion automatically.

This means there are really two answers to “what file format does Kindle use,” depending on who’s asking. Readers receive AZW3 or KFX. Authors submit EPUB. The confusion comes from mixing up these two pipelines, and from outdated guides that still recommend MOBI, a format Amazon stopped accepting years ago.

This glossary covers every file format you’ll encounter as a Kindle author or reader, with clear status labels so you know exactly what’s current and what’s dead.

If you’d rather skip the format headaches entirely, professional book publishing services can handle formatting, conversion, and distribution for you.

Quick-Reference Table

Before the full glossary, here’s a scannable overview of every Kindle-related file format and its current status.

Format	Extension	Status	Who Needs It	Notes
AZW	.azw	Deprecated	Readers (legacy)	Original Kindle format from 2007
AZW3 / KF8	.azw3	Delivery only	Readers	Modern Kindle format; not uploadable to KDP
AZW4	.azw4	Delivery only	Readers	Fixed-layout wrapper for textbooks
DOC / DOCX	.doc, .docx	Accepted upload	Authors	Works but conversion is unreliable
EPUB	.epub	Preferred upload	Authors and readers	The recommended format for KDP and Send to Kindle
HTML	.htm, .html	Accepted upload	Authors	Maximum control, requires web dev skills
KFX	.kfx	Delivery only	Readers	Amazon’s newest format with Enhanced Typesetting
KPF	.kpf	Accepted upload	Authors	Output of Kindle Create tool
MOBI	.mobi	Deprecated	Nobody	Fully retired across all Amazon services
PDF	.pdf	Accepted (print only)	Authors	For print interiors and covers, not ebooks
RTF	.rtf	Accepted upload	Authors	Strips most formatting
TXT	.txt	Accepted upload	Authors	Plain text only, no formatting preserved

Full Glossary of Kindle File Formats

AZW (Kindle Format 7)

Extension: .azw
Status: Deprecated
Who cares: Legacy device owners

AZW was the original Kindle file format, created when Amazon acquired the Mobipocket company in 2005 and adapted their MOBI format into something proprietary. When the first Kindle launched in 2007, AZW was the standard. It supported basic text formatting and Amazon’s DRM but lacked modern features like embedded fonts, HTML5, or CSS3 styling. You won’t encounter AZW files today unless you’re digging through a very old Kindle library.

AZW3 (KF8 / Kindle Format 8)

Extension: .azw3
Status: Active (delivery only)
Who cares: Readers and ebook developers

AZW3, also called KF8 (Kindle Format 8), replaced AZW as the standard Kindle delivery format in late 2011 when the Kindle Fire launched. It was a major leap forward: support for HTML5 and CSS3, embedded fonts, fixed layouts, drop caps, and much better text rendering.

AZW3 is the sweet spot for compatibility. It works on every Kindle from the Paperwhite onward. Amazon’s servers still deliver AZW3 to older devices that can’t handle KFX.

The important thing to understand: AZW3 is a delivery format. KDP does not accept AZW3 uploads, even though it’s Amazon’s own format. This distinction confuses many first-time publishers. For a deeper explanation of Kindle book formatting, see our dedicated guide.

AZW4

Extension: .azw4
Status: Active (niche use)
Who cares: Textbook readers

AZW4 is a fixed-layout wrapper Amazon uses primarily for textbooks and PDF-based content on Kindle. It preserves the exact page layout rather than allowing text to reflow. Most authors will never encounter this format unless they’re publishing academic or highly visual content.

DOC / DOCX

Extension: .doc, .docx
Status: Accepted upload (not recommended)
Who cares: Authors

KDP accepts Microsoft Word files, and for very simple manuscripts (plain text, basic chapter breaks, no images), they convert reasonably well. Once your document includes custom spacing, headers, images, styled text, or tables, things break.

Practitioners on Reddit and publishing forums consistently report that DOCX uploads produce unpredictable results. Spacing gets mangled, tables collapse, and formatting that looked perfect in Word renders poorly on Kindle screens. One common complaint in ebook developer communities is that DOCX conversion strips or ignores CSS styling that authors spent hours perfecting.

The bottom line: if your book is more complex than a basic novel manuscript, skip DOCX and use EPUB.

Enhanced Typesetting

Status: Active feature (not a file format per se)
Who cares: Both authors and readers

Enhanced Typesetting is Amazon’s label for the suite of layout and rendering features that KFX enables. It’s the reason KFX exists. Features include drop caps, improved hyphenation, smoother word spacing, kerning, ligatures, higher-quality image rendering, font alignment, and table formatting.

Amazon applies Enhanced Typesetting only to reflowable books uploaded as EPUB, DOC, DOCX, or HTML. If you upload a properly formatted EPUB, your readers get all of these features automatically on compatible devices. This is one of the practical reasons EPUB produces the best results on KDP.

EPUB

Extension: .epub
Status: Preferred upload format
Who cares: Authors, readers (via Send to Kindle), everyone

EPUB is the open international standard for ebook publishing, maintained by the W3C. It is the recommended upload format for KDP in 2026, and for good reason: it produces the cleanest, most reliable conversion to KFX, gives authors the most control over formatting, and works on every major ebook platform (not just Amazon).

Both EPUB 2 and EPUB 3 are accepted by KDP. EPUB 3 supports more features (audio, video, advanced interactivity), but EPUB 2 has wider compatibility with older reading systems. For most authors publishing standard text-based books, either version works fine.

As one frequently cited point in format discussions: “EPUB won.” It’s the universal ebook standard, and Amazon’s decision to make it the preferred upload format settled the debate.

If you’re planning to publish on Amazon and other platforms simultaneously, EPUB is especially valuable because you prepare one file and distribute it everywhere.

HTML

Extension: .htm, .html
Status: Accepted upload
Who cares: Technically skilled authors

KDP accepts HTML uploads, which give you maximum control over how your ebook looks. You can specify every CSS rule, every heading hierarchy, every image placement. The downside is that this requires real web development skills. Most authors are better served by EPUB, which is itself built on HTML and CSS but packaged in a more manageable container.

KFX

Extension: .kfx
Status: Active (delivery only)
Who cares: Readers

KFX is Amazon’s newest and most advanced ebook format, introduced in 2015. It’s what readers actually receive when they buy a Kindle book today (assuming their device supports it). KFX isn’t a single file but a bundle of assets, built on Amazon’s proprietary Ion binary format.

Key capabilities include intelligent justification, Bookerly font optimizations, Enhanced Typesetting, smaller file sizes, and the best reading experience Amazon offers. Amazon’s servers decide whether to deliver KFX or AZW3 based on the reader’s device and app version. A 2013 Kindle Paperwhite might receive AZW3, while a 2026 model gets KFX.

Authors never create KFX directly. The format exists entirely on Amazon’s side of the pipeline.

KPF (Kindle Package Format)

Extension: .kpf
Status: Accepted upload
Who cares: Authors using Kindle Create

KPF is the output format of Kindle Create, Amazon’s free desktop formatting tool. It’s essentially a packaged file that preserves the layout decisions you made in Kindle Create, and KDP accepts it as a direct upload.

KPF works particularly well for image-heavy books or fixed-layout content where page design matters. If you’re publishing a children’s book with illustrations on every page, KPF from Kindle Create is a solid choice.

A common confusion: KPF and KFX are related but not the same. KPF is what you upload (your input file). KFX is what Amazon delivers to readers (the output). Think of KPF as the recipe you hand to Amazon’s kitchen, and KFX as the dish that arrives at the reader’s table.

MOBI

Extension: .mobi, .prc
Status: Fully deprecated
Who cares: Nobody (as of 2025)

MOBI was created by the Mobipocket company in 2000 and became the foundation of Kindle’s early file formats after Amazon’s acquisition. For over a decade, it was the default format for Kindle ebooks.

It’s now dead.

Amazon stopped accepting MOBI uploads for reflowable ebooks on August 1, 2021. Send to Kindle stopped delivering MOBI files to devices in August 2022. The final holdout, fixed-layout MOBI ebooks, was retired on March 18, 2025.

MOBI was never designed for modern typography. As technical analyses have noted, it lacked native support for complex CSS layouts, media queries, SVG, embedded modern fonts, hyphenation dictionaries, and dark mode styling. Its limitations were severe even by 2010 standards.

If you encounter instructions telling you to submit a .mobi file, those instructions are outdated. For a full comparison, read our EPUB vs. MOBI format guide.

Existing MOBI files in your Kindle library still work, but for anything new, convert to EPUB. Calibre (free, open-source software) handles this conversion reliably.

PDF

Extension: .pdf
Status: Accepted for print, not recommended for ebook
Who cares: Authors (print pipeline)

PDF is a fixed-layout format. It preserves exact page dimensions, font positioning, and image placement. That makes it perfect for print book interiors and covers submitted to KDP, but terrible for ebook manuscripts.

The problem is simple: PDFs don’t reflow. On a small Kindle screen, text stays at its original size rather than adjusting to the display. Readers end up pinching and scrolling instead of reading comfortably. For ebook purposes, EPUB is always the better choice.

KDP currently supports PDF uploads for ebook in a limited set of languages: English, French, German, Italian, Portuguese, Spanish, Catalan, Galician, and Basque. Even where supported, the reading experience is inferior to a properly formatted EPUB.

RTF and TXT

Extensions: .rtf, .txt
Status: Accepted upload
Who cares: Authors with very basic manuscripts

KDP technically accepts both RTF (Rich Text Format) and TXT (plain text) files. In practice, these formats strip all meaningful formatting. No embedded images, no styled headings, no custom fonts. They exist as fallback options, not as serious publishing tools.

Upload Formats vs. Delivery Formats: Why the Distinction Matters

This is the single most important concept in understanding what file format Kindle uses, and most guides fail to explain it clearly.

There are three stages in the Kindle format pipeline:

1. Upload format (what authors submit to KDP): EPUB, DOCX, KPF, HTML, PDF, RTF, or TXT
2. Conversion (what Amazon does internally): KDP’s servers transform your upload into Kindle-optimized formats
3. Delivery format (what readers receive on their device): KFX on modern devices, AZW3 on older ones

Authors never touch AZW3 or KFX. They never need to create those files. The entire conversion happens automatically on Amazon’s servers after you upload your manuscript.

This is why EPUB is the best upload choice. It converts cleanly because EPUB and KFX share similar underlying structures (both rely on HTML and CSS). A well-formatted EPUB passes through Amazon’s conversion with minimal surprises.

DOCX, by contrast, requires Amazon to interpret Word’s proprietary formatting and translate it into web standards. That translation is where spacing breaks, tables collapse, and carefully designed layouts fall apart. Practitioners on Adobe Community forums report that even fixed-layout EPUBs exported from InDesign can cause conversion failures in Kindle Previewer 3, so the recommendation for most text-based books is to stick with reflowable EPUB.

For a full walkthrough of the KDP upload process, our self-publishing on Amazon KDP guide covers every step.

Cover Image Format Requirements

Ebook covers and print covers have completely different format requirements. This catches many first-time authors off guard.

Ebook covers:

• Accepted formats: JPG or TIFF only (no PNG)
• Recommended dimensions: 1600 x 2560 pixels (1.6:1 aspect ratio)
• Maximum file size: 50 MB
• Color profile: RGB only (Kindle does not support CMYK for ebook covers)

Print covers:

• Format: Single PDF wrapping the back cover, spine, and front cover
• Resolution: 300 DPI minimum
• Color profile: CMYK preferred for accurate color reproduction
• Must include bleed area per KDP’s template specifications

The distinction between RGB (screen) and CMYK (print) trips up many authors who use one cover file for both formats. For detailed guidance on dimensions, typography, and design best practices, see our book cover design glossary.

The MOBI-to-EPUB Timeline: A Brief History

Understanding how Kindle formats evolved explains why so much outdated advice still circulates online.

• 2000: Mobipocket creates the MOBI format
• 2005: Amazon acquires Mobipocket
• 2007: First Kindle launches using AZW, a proprietary adaptation of MOBI
• 2011: Kindle Format 8 (AZW3/KF8) introduced with the Kindle Fire, adding HTML5 and CSS3 support
• 2015: KFX introduced, bringing Enhanced Typesetting to Kindle devices
• August 2021: KDP stops accepting MOBI uploads for reflowable ebooks
• August 2022: Send to Kindle drops MOBI support and adds native EPUB support
• March 2025: KDP stops accepting MOBI for fixed-layout ebooks, completing the deprecation
• 2026: EPUB is the universal standard across Amazon’s entire ecosystem

The ebook developer community has been blunt about this transition. As one widely shared assessment puts it: “MOBI is no longer a debate. It’s a historical footnote.” If you’re still choosing between formats in 2026, the answer is EPUB, full stop.

Which Format Should You Use?

Different situations call for different formats. Here’s a straightforward decision matrix.

Publishing an ebook on KDP?
Upload EPUB. It’s the recommended format and produces the highest-quality conversion.

Using Kindle Create for a visual book?
Upload KPF. It preserves your layout decisions from Kindle Create and handles image-heavy or fixed-layout content well.

Uploading a print book interior?
Use PDF. It’s the only format that preserves exact page layouts for physical printing.

Sideloading a personal ebook to your Kindle?
Use EPUB via Send to Kindle. Amazon converts it to AZW3 automatically for a smooth reading experience.

Have old MOBI files you want to read?
Convert them to EPUB using Calibre (free, open-source). Existing MOBI files already in your Kindle library still work, but you can’t send new MOBI files to your device.

Writing a simple novel manuscript and want the easiest path?
DOCX technically works, but expect formatting quirks. If you care about how your book looks (and you should), take the extra step to convert to EPUB before uploading. Our manuscript formatting guide walks through preparing your document properly.

Publishing across multiple platforms, not just Amazon?
EPUB is the universal standard. One well-formatted EPUB file works for KDP, Apple Books, Barnes & Noble, Kobo, and virtually every other retailer.

Send to Kindle: What Readers Need to Know

Amazon’s Send to Kindle service lets readers send personal documents to their Kindle devices via email or the Send to Kindle app. As of 2026, the supported formats for Send to Kindle include EPUB, PDF, DOCX, TXT, RTF, and HTML.

When you send an EPUB file to your Kindle, Amazon converts it into Kindle’s internal format (usually AZW3) for proper rendering. The conversion happens automatically and the file appears in your Kindle library alongside purchased books.

The maximum upload size for KDP is 650 MB for both print and ebook files, though most ebooks fall well under that limit.

Frequently Asked Questions

Can I upload AZW3 or KFX files directly to KDP?

No. Despite being Amazon’s own formats, AZW3 and KFX are delivery formats, not upload formats. KDP accepts EPUB, DOCX, KPF, HTML, PDF (for print), RTF, and TXT. Amazon’s servers handle the conversion to AZW3/KFX automatically.

Is MOBI still supported anywhere in Amazon’s ecosystem?

No. Amazon completed the MOBI deprecation in March 2025 when it stopped accepting fixed-layout MOBI files, the last holdout. MOBI books already in your library still function, but you cannot upload or send new MOBI files. Convert any MOBI files to EPUB using free tools like Calibre.

What’s the difference between KPF and KFX?

KPF (Kindle Package Format) is the file Kindle Create produces. It’s what you upload to KDP. KFX is what Amazon delivers to readers after converting your upload. They’re related technically but serve different roles in the publishing pipeline.

Why does my DOCX file look different on Kindle than it does in Word?

Amazon must convert Word’s proprietary formatting into web-based standards (HTML/CSS) during the KDP upload process. Complex formatting like custom spacing, styled headers, tables, and image positioning often breaks during this translation. EPUB gives you direct control over the HTML and CSS, which is why it converts more reliably.

What format should I use for a children’s picture book on Kindle?

Fixed-layout content works best with KPF generated by Kindle Create. Standard reflowable EPUB is designed for text-heavy books where content reflows to fit different screen sizes. Picture books need exact page layouts, which KPF handles well. Practitioners in ebook development communities recommend avoiding reflowable formats for books whose pages are primarily images.

Can I send EPUB files directly to my Kindle?

Yes. Amazon added native EPUB support to Send to Kindle in August 2022. You can email EPUB files to your Kindle’s unique email address or use the Send to Kindle app. Amazon converts the file to its internal format automatically.

Does the file format affect my book’s eligibility for Enhanced Typesetting?

Yes. Amazon applies Enhanced Typesetting only to reflowable books uploaded as EPUB, DOC, DOCX, or HTML. If your book qualifies, readers get improved hyphenation, kerning, drop caps, ligatures, and better image rendering. Uploading a clean EPUB gives you the best chance of full Enhanced Typesetting support.

What cover image format does Kindle accept?

For ebooks, KDP accepts JPG and TIFF only. The recommended size is 1600 x 2560 pixels with an RGB color profile. File size must stay under 50 MB. Print covers require a separate PDF file. For comprehensive cover specs, see our book cover design guide.

Get Your Formatting Right the First Time

Understanding what file format Kindle uses is the foundation, but executing it correctly across ebook, print, and audio takes real attention to detail. If you’d rather focus on writing while someone else handles the technical side, Alpaca Authors offers book publishing packages that include editorial, cover design, ISBN setup, formatting, and distribution to 40+ platforms, Amazon included.