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Guide

MOBI Format: Kindle Ebook Format History, AZW3 Successor & Conversion

PC By Pablo Cirre •

MOBI (Mobipocket) Kindle Ebook Format — Complete Guide

What Is MOBI?

MOBI (also called Mobipocket) is an ebook file format originally developed by Mobipocket SA, a French company founded in 2000. Amazon acquired Mobipocket in 2005 and used the format as the basis for Kindle ebooks. For nearly a decade (2007–2015), MOBI was the primary Kindle ebook format, making it one of the most widely distributed ebook formats in history.

Amazon has since deprecated MOBI, transitioning to the superior AZW3 (KF8) format, and as of April 2022, Amazon's "Send to Kindle" service no longer accepts MOBI files. However, hundreds of millions of MOBI books remain in existence on Kindle devices, Calibre libraries, and ebook archives.

Technical Foundation

Mobipocket Format Origins

Mobipocket was derived from the Open eBook (OEB) standard (a precursor to EPUB), extended with proprietary compression and DRM layers. The format was designed for Palm OS PDAs and feature phones of the early 2000s — hence its emphasis on small file sizes and resource efficiency.

PalmDOC Base

The underlying data structure is PalmDOC, a format for the Palm OS PDA database system:

  • Data stored in PalmDOC records (4096-byte blocks)
  • HTML content (simplified XHTML) compressed using the PalmDOC LZ77-variant compression
  • Index records for chapter navigation

MOBI Header

MOBI files begin with a PalmDOC header followed by an extended MOBI header with fields:

  • identifier — "MOBI" (4 bytes)
  • header_length — total header size
  • mobi_type — type code (2 = MOBI Book)
  • text_encoding — UTF-8 (65001) or Windows-1252 (1252)
  • unique_id — random identifier for this book file
  • file_version — MOBI version (6 = standard MOBI; 8 = KF8/AZW3)
  • first_image_index — record index of the first embedded image
  • exth_flags — indicates presence of EXTH header (metadata)
  • min_version — minimum reading app version required

EXTH Block (Extension Header)

Metadata is stored in the EXTH block — a sequence of tagged records:

  • Tag 100: Author
  • Tag 101: Publisher
  • Tag 103: Description (book blurb)
  • Tag 104: ISBN
  • Tag 105: Subject/Genre
  • Tag 106: Publication date
  • Tag 109: Copyright holder
  • Tag 200: Cover image record index (thumbnail)
  • Tag 201: Cover image record index (full resolution)
  • Tag 501: CDETYPE ('EBOK' = ebook; 'PDOC' = personal document)
  • Tag 503: Updated title
  • Tag 524: Language

Content Format in MOBI

MOBI content uses a simplified, heavily restricted HTML dialect:

  • Supported tags: <a>, <b>, <big>, <blockquote>, <br>, <center>, <code>, <dd>, <dl>, <dt>, <em>, <font>, <h1>–<h6>, <i>, <img>, <li>, <mbp:pagebreak>, <ol>, <p>, <s>, <small>, <strike>, <strong>, <sub>, <sup>, <table> (limited), <td>, <tr>, <u>, <ul>
  • No div elements with complex CSS positioning
  • No JavaScript
  • No audio/video embedding
  • Limited CSS: font properties, text alignment, margins, simple colours
  • Images: JPEG or GIF only (no PNG in original MOBI; later versions allow PNG)

This restriction to basic HTML makes MOBI easily portable across low-powered devices but severely limits design complexity compared to EPUB 3 or AZW3.

MOBI vs. AZW3 (KF8)

Amazon introduced KF8 (Kindle Format 8) in 2011, branded as the AZW3 file extension:

Feature MOBI AZW3 (KF8)
Base format PalmDOC + MOBI extension MOBI container with embedded EPUB 3-like content
HTML version Simplified HTML 3.2-level Full HTML5
CSS support CSS 1.5 (subset) CSS 3 (most properties)
Fonts System fonts only Embedded OTF/TTF fonts
Tables Very limited Full table support
Drop caps No Yes
Images JPEG/GIF JPEG, PNG, SVG
Fixed layout No Yes (children's books, manga)
Kindle Textbooks No Yes
File size Smaller (limited features) Larger (richer content)
Device support All Kindle hardware Kindle Fire, Paperwhite (2012+), Kindle apps

Dual-Format MOBI Files

Amazon distributed books as "MOBI files" containing both MOBI and KF8 content packaged together — the Kindle device automatically selected the appropriate format based on its capabilities. This dual-format file was sometimes called "KFX" informally, though KFX is actually a separate next-generation Kindle format.

Amazon's MOBI Deprecation (2022)

On January 4, 2022, Amazon announced:

  • The "Send to Kindle" email service would stop accepting .mobi files after April 2022
  • Kindle apps and devices already supported EPUB as of 2021
  • EPUB and PDF are now the recommended formats for personal document delivery

Despite deprecation of the delivery channel, existing MOBI files continue to function on Kindle hardware and apps. The MOBI format itself is not "dead" — it simply receives no further development.

MOBI DRM (Mobipocket DRM)

Kindle-purchased MOBI books use Amazon's proprietary DRM:

  • Device-specific encryption tied to the Amazon account's registered Kindle serial numbers
  • MOBI files purchased on Amazon cannot be opened outside the Kindle ecosystem
  • DRM-removal tools exist but their use is legally restricted in most jurisdictions

DRM-free MOBI: Available from Project Gutenberg (public domain), Smashwords, and some independent authors. These files are portable across any MOBI-compatible reader.

Software That Opens MOBI Files

Software Platform Notes
Kindle App Windows, Mac, iOS, Android Native; DRM-protected files
Calibre Windows, Mac, Linux Free; library management + conversion
Moon+ Reader Pro Android Full MOBI support
FBReader Android, Linux Open-source; MOBI support
Kindle e-readers Hardware Native
KOReader Kindle hardware, Android Open-source reader with MOBI support

Converting MOBI Files

MOBI → EPUB

The most common conversion — produces a format readable on all non-Kindle devices:

  • Calibre performs excellent MOBI → EPUB conversion with minimal information loss
  • Complex tables or advanced formatting may simplify during conversion
  • DRM-protected files cannot be converted without DRM removal

MOBI → PDF

Useful for printing or sharing with PDF readers:

  • Reflowable MOBI → PDF requires choosing a page size; fixed margins applied
  • Content flows to fit the specified page dimensions

MOBI → AZW3

For upgrading legacy MOBI to Kindle's modern format:

  • Calibre converts MOBI → AZW3 directly
  • More formatting features available in AZW3 output than original MOBI could represent

EPUB → MOBI

For compatibility with very old Kindle hardware (Kindle 1, 2, DX):

  • Calibre converts EPUB → MOBI reliably
  • Amazon's KDP publishing platform accepts EPUB and converts to the appropriate Kindle format automatically
  • Consider using AZW3 rather than MOBI for better quality on modern Kindles

The Kindle Format Ecosystem (Timeline)

Year Format Notes
2007 MOBI Original Kindle format; Palm DOC base
2011 AZW3 (KF8) HTML5/CSS3; modern ebook typography
2015 KFX Advanced typography, Bookerly font, page flip; Kindle Paperwhite (2015+)
2021 EPUB native Kindle apps accept EPUB via Send to Kindle
2022 MOBI deprecated No longer accepted by Send to Kindle service

Understanding this timeline explains why converting MOBI → EPUB or AZW3 is valuable: the original MOBI format's limitations (no CSS 3, no embedded fonts, no proper table support) produced inferior ebook rendering compared to modern formats.

Related conversions

Document conversions that follow this topic naturally:

Related conversions

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Frequently Asked Questions

Yes, existing MOBI files already on your Kindle or in your Kindle library continue to work. Amazon only stopped accepting new MOBI uploads via Send to Kindle in 2022. Old MOBI purchases remain accessible in your Amazon library. New ebooks should use EPUB or PDF for Send to Kindle, or AZW3 for direct Kindle publishing.

Sim, existing MOBI arquivos already on your Kindle ou in your Kindle library continue to work. Amazon only stopped accepting new MOBI uploads via enviar to Kindle in 2022. Old MOBI purchases remain accessible in your Amazon library. New ebooks should usar EPUB ou PDF para enviar to Kindle, ou AZW3 para direct Kindle publishing.

Ja, existing MOBI Dateien already on your Kindle oder in your Kindle library continue to work. Amazon only stopped accepting new MOBI uploads via senden to Kindle in 2022. Old MOBI purchases remain accessible in your Amazon library. New ebooks should verwenden EPUB oder PDF für senden to Kindle, oder AZW3 für direct Kindle publishing.

Sí, existing MOBI archivos already on your Kindle o in your Kindle library continue to work. Amazon only stopped accepting new MOBI uploads via enviar to Kindle in 2022. Old MOBI purchases remain accessible in your Amazon library. New ebooks should usar EPUB o PDF para enviar to Kindle, o AZW3 para direct Kindle publishing.

Send <strong>PDF</strong> when the document is final and the layout must be preserved exactly (contracts, invoices, certificates). Send <strong>DOCX</strong> when reviewers need to edit, comment, or track changes. Many teams send both: PDF as the canonical version + DOCX for editable feedback. PDF/A is the right pick for legal archival (ISO 19005).

Related but different. MOBI is the original Mobipocket format acquired by Amazon. AZW was Amazon's first branded Kindle format (essentially MOBI with Amazon DRM). AZW3 (also called KF8) is a much more capable successor using HTML5/CSS3, embedded fonts, and richer formatting. All three use similar container structures but AZW3 is significantly more advanced.

Related mas different. MOBI is the original Mobipocket formato acquired by Amazon. AZW was Amazon's first branded Kindle formato (essentially MOBI com Amazon DRM). AZW3 (also called KF8) is a much more capable successor usando HTML5/CSS3, embedded fonts, e richer formatoting. All three usar similar container structures mas AZW3 is significantly more advanced.

Related aber different. MOBI is the original Mobipocket Format acquired by Amazon. AZW was Amazon's first branded Kindle Format (essentially MOBI mit Amazon DRM). AZW3 (also called KF8) is a much more capable successor using HTML5/CSS3, embedded fonts, und richer Formatting. All three verwenden similar Container structures aber AZW3 is significantly more advanced.

Related pero different. MOBI is the original Mobipocket formato acquired by Amazon. AZW was Amazon's first branded Kindle formato (essentially MOBI con Amazon DRM). AZW3 (also called KF8) is a much more capable successor using HTML5/CSS3, embedded fonts, y richer formatoting. All three usar similar contenedor structures pero AZW3 is significantly more advanced.

Round-tripping between similar formats (DOCX ↔ ODT, DOCX → PDF) is generally safe. Round-tripping with format-specific features (Word macros, complex tables, footnotes) often loses fidelity. Embedded fonts survive only if both source and target support font embedding (PDF yes, DOCX yes, plain HTML no). Always preview the result before deleting the original.

Use Calibre (free, open-source): Import the MOBI file, right-click it in your library, and select "Convert books." Choose EPUB as output format. Calibre maps MOBI metadata (title, author, cover) and content to EPUB. Complex MOBI features (simplified tables, custom fonts) may look slightly different in EPUB; review the output after conversion.

Use Calibre (free, open-source): Import the MOBI file, right-click it in your library, e select "Convert books." Choose EPUB as output formato. Calibre maps MOBI metadata (title, author, cover) e content to EPUB. complexo MOBI features (simplified tables, custom fonts) may look slightly different in EPUB; review the output depois conversion.

Use Calibre (free, open-source): Import the MOBI file, right-click it in your library, und select "Convert books." Choose EPUB as output Format. Calibre maps MOBI metadata (title, author, cover) und content to EPUB. Complex MOBI features (simplified tables, custom fonts) may look slightly different in EPUB; review the output nach conversion.

Use Calibre (free, open-source): Import the MOBI file, right-click it in your library, y select "Convert books." Choose EPUB as output formato. Calibre maps MOBI metadata (title, author, cover) y content to EPUB. Complex MOBI features (simplified tables, custom fonts) may look slightly different in EPUB; review the output después conversion.

If the PDF contains real text (not scanned images), <code>pdftotext</code> from poppler-utils or <a href="/convert/pdf-to-txt">PDF to TXT</a> works in seconds. If the PDF is a scanned image, you need OCR — Tesseract is the open-source standard. KaijuConverter's PDF tools auto-detect text-vs-image PDFs and route accordingly.

Yes, via the free Kindle app for iOS. The Kindle app reads MOBI files (and AZW, AZW3, PDF) from your Amazon library. For personal MOBI files not purchased from Amazon, use Calibre to convert them to EPUB and open with Apple Books, or to PDF and open with Files app. Direct MOBI import without Kindle app requires a third-party reader like Moon+ Reader.

Sim, via the grátis Kindle app para iOS. The Kindle app reads MOBI arquivos (and AZW, AZW3, PDF) de your Amazon library. para personal MOBI arquivos not purchased de Amazon, usar Calibre to converter them to EPUB e abrir com Apple Books, ou to PDF e abrir com arquivos app. Direct MOBI import sem Kindle app requires a third-party reader like Moon+ Reader.

Ja, via the kostenlos Kindle app für iOS. The Kindle app reads MOBI Dateien (and AZW, AZW3, PDF) von your Amazon library. für personal MOBI Dateien not purchased von Amazon, verwenden Calibre to umwandeln them to EPUB und öffnen mit Apple Books, oder to PDF und öffnen mit Dateien app. Direct MOBI import ohne Kindle app requires a third-party reader like Moon+ Reader.

Sí, via the gratis Kindle app para iOS. The Kindle app reads MOBI archivos (and AZW, AZW3, PDF) de your Amazon library. para personal MOBI archivos not purchased de Amazon, usar Calibre to convertir them to EPUB y abrir con Apple Books, o to PDF y abrir con archivos app. Direct MOBI import sin Kindle app requires a third-party reader like Moon+ Reader.

Light edits (annotations, signatures, form fields) are fine in any PDF reader. Structural edits (changing paragraphs, replacing images) are awkward — PDF is a presentation format, not an editing format. The robust workflow is: keep the source DOCX/MD/HTML as the master, regenerate the PDF when changes are needed. Tools that "edit PDFs" reverse-engineer the layout and frequently break it.