Amazon's ebook format history is a study in vertical integration. When Jeff Bezos launched the first Kindle in 2007, he didn't adopt the open EPUB standard — he bought Mobipocket, took their format, added DRM, and called it AZW. Every format since has been an iteration on that decision: build a proprietary stack, lock customers to the Kindle ecosystem, and convert everything else on the way in.

Understanding this timeline matters because millions of ebooks exist in these formats, and the format you have determines what you can do with it. A DRM-free MOBI converts cleanly to EPUB. A DRM-protected AZW file is locked to Amazon forever. An AZW3 from Calibre works on Kindle but can't be read on Kobo. The format isn't just a container — it's a statement about who controls the content.

This guide covers every format Amazon has used, what each one actually is under the hood, and the practical conversion paths that exist in 2026.

The Mobipocket Acquisition (2005)

In April 2005, Amazon acquired Mobipocket SA, a French company that had built an ebook ecosystem around Palm OS devices. The Mobipocket format (.mobi, .prc) used PalmDOC compression and a subset of HTML for content, with a binary header containing metadata. It was designed for devices with 160x160 pixel screens, 2MB of RAM, and 8MHz processors.

The acquisition gave Amazon three things: a working ebook format, a DRM system, and reader software that ran on Palm, Windows Mobile, Symbian, and BlackBerry. When the Kindle launched two years later, it used Mobipocket's format with Amazon's own DRM layer on top. The Mobipocket Reader app continued running on non-Kindle platforms until Amazon shut it down in 2016, cutting off the last non-Amazon devices that could read the format.

MOBI/PRC: The Palm Heritage

The MOBI format (file extensions .mobi and .prc) is based on the Palm Database format (PDB). The file structure consists of a PalmDOC header, a MOBI header with extended metadata, and compressed HTML content split into 4096-byte records. Images are appended as binary records after the text.

MOBI Technical Internals

The content model is simple: a single long HTML document (no separate chapter files like EPUB), compressed with either PalmDOC (LZ77-variant) or Huffman/CDIC compression. The HTML subset is limited — basic formatting tags (<b>, <i>, <h1>-<h6>, <p>, <img>, <table>), inline images via recindex attributes, and page breaks via <mbp:pagebreak/>.

The MOBI header contains EXTH (Extended Header) records for metadata: title, author, publisher, ISBN, subject, description, cover image index, and thumbnail image index. This is a flat key-value system, less structured than Dublin Core in EPUB but functional for library management. The cover image is stored as one of the binary image records, referenced by index in the EXTH header.

Maximum file size for MOBI is theoretically unlimited, but the 4096-byte record structure and text offsets stored in 32-bit integers create practical limits. Books over 100MB with many images can hit edge cases in older Kindle firmware.

Why MOBI Is Dead

MOBI's limitations were severe even by 2010 standards: no embedded fonts, no CSS beyond basic inline styles, no SVG, no MathML, no audio, no video, limited table rendering, and a single-document structure that prevented clean chapter navigation. Amazon responded by building KF8 (AZW3) in 2011 and steadily phasing MOBI out.

The deprecation timeline: Amazon stopped accepting MOBI uploads to the Kindle Store in June 2021. Send-to-Kindle stopped delivering MOBI files to devices in August 2022. As of 2026, MOBI files already on a Kindle still work, but there's no supported way to get new ones onto devices. If you have MOBI files, convert them to EPUB now. The format is functionally dead.

AZW: MOBI with Amazon's DRM

AZW (.azw) is not a distinct format — it's a MOBI file wrapped in Amazon's DRM scheme. The content structure, compression, and metadata are identical to MOBI. The difference is a DRM layer that encrypts the content and ties it to an Amazon account. When you purchase a book from the Kindle Store pre-2012, the file you received was AZW.

DRM-free AZW files are byte-for-byte identical to MOBI files except for the extension and a DRM flag in the header set to "none." Many publishers who distributed through the Kindle Store without DRM shipped AZW files that could be renamed to .mobi and read by any Mobipocket-compatible reader.

AZW is a legacy format. Amazon transitioned Kindle Store purchases to AZW3 and later KFX. You'll still encounter AZW files from old Kindle libraries (pre-2012 purchases) and from early DRM-free publishers. Converting to EPUB works for DRM-free files; DRM-protected AZW cannot be converted by any legitimate tool.

AZW3 / KF8: The Modern Kindle Format

Kindle Format 8 (KF8), with the file extension .azw3, launched with the Kindle Fire in November 2011. It was Amazon's answer to EPUB 3, built to support the rich content that MOBI couldn't handle.

KF8 Architecture

Internally, AZW3 uses a MOBI container with an embedded KF8 section. Early AZW3 files (2011-2013) actually contained two complete books: a MOBI version for older Kindles and a KF8 version for newer devices. The Kindle firmware would detect the device capability and render the appropriate version. Later, Amazon dropped the dual-format approach as older devices fell out of use.

The KF8 content model is HTML5 and CSS3, organized as separate XHTML fragments (like EPUB chapters) linked via a fragment table in the binary header. CSS support includes @font-face, text-indent, margin/padding, float, border, background-color, and text-transform. Amazon added proprietary CSS extensions: -amzn-max-crop-percentage, -amzn-trim-color, and -webkit prefixed properties for their WebKit-based renderer.

AZW3 supports embedded fonts (TTF, OTF), SVG images, fixed-layout rendering, and print replica mode (embedding a PDF-like document in a Kindle container). It does not support JavaScript, MathML (Amazon renders math as images), or audio/video embedding.

AZW3 vs EPUB: Nearly Identical Under the Hood

The irony of AZW3 is that it's structurally almost identical to EPUB 3. Both use HTML5 content documents, CSS3 styling, embedded fonts, and XML metadata. The differences are container format (EPUB uses ZIP, AZW3 uses MOBI-derived binary container), DRM implementation, and some CSS extensions. This similarity is why Calibre converts between EPUB and AZW3 so cleanly — the content translates 1:1 with minimal transformation.

Converting EPUB to AZW3: straightforward, high fidelity. Complex CSS may need adjustment for Amazon's renderer, but text, images, metadata, and structure transfer cleanly. Converting AZW3 to EPUB: equally clean for DRM-free files. The content model maps directly. DRM-protected AZW3 files from the Kindle Store cannot be converted.

KFX: Enhanced Typesetting (2015-Present)

KFX is Amazon's latest proprietary format, introduced in 2015 for Kindle Store purchases. It's not a single file but a bundle of files (with .azw extensions in the device's storage) using Amazon's Ion binary format for content serialization.

What KFX Adds

KFX's "enhanced typesetting" includes: improved justification and hyphenation (character-level spacing instead of word-level), kerning pairs for common character combinations, ligature support, soft hyphens, widow/orphan control, drop caps with proper text wrapping, Bookerly and Ember typeface optimization, and ragged-right margins for CJK text.

KFX also enables: X-Ray (character and term lookup), Word Wise (simple definitions above difficult words), Page Flip (thumbnail page scrubbing), and image zoom pop-overs. These features are tightly coupled to Amazon's reading app and don't translate to other formats — they're Kindle-exclusive value-adds.

The typographic improvements are genuinely good. Side-by-side, a KFX-rendered page looks noticeably better than the same content in AZW3 — tighter word spacing, fewer rivers in justified text, better hyphenation. Amazon hired real typographers for this, and it shows.

Creating KFX: You Don't

KFX is not an authoring format. You cannot create KFX files with any publicly available tool. When you upload an EPUB or DOCX to KDP (Kindle Direct Publishing), Amazon's backend converts it to KFX for delivery to devices. Kindle Create, Amazon's free layout tool, exports to KPF (Kindle Package Format), which KDP then converts to KFX during the publishing process.

Kindle Previewer 3 can render a simulated KFX view of your manuscript, showing how Amazon's enhanced typesetting will affect the final output. This is the closest you get to "previewing" KFX before publishing. If the preview shows formatting issues, fix them in your EPUB/DOCX source — you can't edit KFX directly.

Send-to-Kindle Now Accepts EPUB (2022)

In April 2022, Amazon added EPUB support to Send-to-Kindle — the feature that lets you email documents to your Kindle's @kindle.com address. This was the single most significant change in Kindle format policy since the original launch.

What This Means in Practice

Before April 2022, getting a non-Amazon ebook onto Kindle required converting to MOBI or AZW3 using Calibre or a similar tool. Now you email the EPUB to your @kindle.com address, Amazon converts it server-side (likely to KFX with enhanced typesetting), and it appears in your Kindle library with full syncing, bookmarking, and highlighting support.

Amazon simultaneously deprecated MOBI for Send-to-Kindle. MOBI files sent after August 2022 are silently rejected — they don't appear on devices. The message was clear: EPUB is now the personal document format for Kindle.

Supported EPUB features via Send-to-Kindle: reflowable text, embedded images, table of contents, basic CSS, cover images. Limitations: DRM-protected EPUBs are rejected, extremely complex CSS may render differently than intended, and fixed-layout EPUBs may be reflowed or poorly formatted. For novels and standard non-fiction, it works excellently.

Kindle Create and Kindle Previewer

Amazon provides two free tools for publishers targeting Kindle:

Kindle Create

Kindle Create is a desktop application (Windows/Mac) for formatting manuscripts before KDP upload. It imports DOCX or PDF files, provides a visual editor for chapter styling (themes for chapter headings, first-paragraph formatting, drop caps), and exports to KPF (Kindle Package Format). KPF is a ZIP containing structured content that KDP processes into the final KFX.

Kindle Create is useful if you're starting from a Word document and want Kindle-optimized formatting without learning EPUB. It handles: chapter detection, table of contents generation, image placement, print replica mode (for PDF imports), and text pop-up overlays. It does not replace a full ebook editor like Sigil — you can't directly edit HTML/CSS, and the formatting options are limited to Amazon's predefined themes.

Kindle Previewer 3

Kindle Previewer 3 simulates how your ebook will render on Kindle devices and apps. It accepts EPUB, DOCX, KPF, MOBI, and HTML as input and shows the output as it would appear on Kindle Paperwhite (e-ink), Kindle Fire (tablet), and Kindle for iOS/Android. The previewer includes an enhanced typesetting simulation showing KFX-level rendering.

The previewer also flags potential issues: images below recommended resolution (300 DPI for images expected to fill the screen), tables that may not render well on small screens, and content that may trigger display problems on specific devices. Run every ebook through Kindle Previewer before uploading to KDP — it catches formatting problems that aren't visible in Calibre or Sigil.

Practical Conversion Paths in 2026

Given Amazon's format history, here's what actually works for conversion:

  • MOBI to EPUB — Clean conversion, high fidelity. MOBI's simpler formatting maps well to EPUB. Convert here
  • MOBI to AZW3 — Works, but why? If you have MOBI, convert to EPUB and let Send-to-Kindle handle the rest. Convert here if you specifically need AZW3
  • AZW3 to EPUB — Excellent for DRM-free files. The content model is nearly identical. Convert here
  • EPUB to AZW3 — The standard path for getting content onto older Kindles that don't support Send-to-Kindle EPUB. Convert here
  • EPUB to MOBI — Only needed for very old Kindles (pre-2012 firmware). Convert here but consider if EPUB via email is better
  • KFX — Cannot be converted. Amazon proprietary, generated server-side only

Amazon's format history is a 20-year arc from acquiring Mobipocket to reluctantly accepting EPUB. The proprietary stack — MOBI, AZW, AZW3, KFX — was built to lock customers into the Kindle ecosystem. It worked: Kindle has the largest ebook market share globally. But the practical landscape shifted in 2022 when Amazon accepted EPUB for personal documents.

For publishers, the format question is settled: upload EPUB to KDP, and Amazon generates the optimal format for each device. For readers, EPUB is the universal starting point — send it to Kindle via email, or read it on any other platform directly. The only format conversion still routinely needed is EPUB to AZW3 for sideloading onto older Kindles, and that's a clean, lossless conversion.