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 sizemobi_type— type code (2 = MOBI Book)text_encoding— UTF-8 (65001) or Windows-1252 (1252)unique_id— random identifier for this book filefile_version— MOBI version (6 = standard MOBI; 8 = KF8/AZW3)first_image_index— record index of the first embedded imageexth_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
.mobifiles 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: