Mobipocket E-book
MOBI is an eBook format created by Mobipocket SA (acquired by Amazon in 2005), built on the PalmDOC/PDB container. It stores formatted text, images, bookmarks, and DRM data, largely replaced by AZW3 (KF8) and KFX on modern Kindle devices.
Mobipocket format requires proprietary rendering engine and DRM handling not available in browser.
Looking to convert? Try a related format:
Common questions
Can I still use MOBI files on my Kindle?
Yes, existing MOBI files work on all Kindle devices and apps. However, Amazon stopped accepting MOBI uploads to Kindle Direct Publishing in 2022 — new books must be submitted as EPUB or DOCX. For personal sideloading, MOBI still works via USB transfer.
How do I convert MOBI to EPUB for free?
Install Calibre (free, cross-platform) and run: ebook-convert input.mobi output.epub. Calibre preserves the table of contents, cover image, and chapter structure. This only works on DRM-free MOBI files.
What replaced the MOBI format?
Amazon replaced MOBI with AZW3 (KF8) starting in 2011, which supports HTML5, CSS3, and embedded fonts. The latest Kindle format is KFX, which adds enhanced typesetting and Kindle Page Flip. The open-standard equivalent is EPUB3.
Why are MOBI files larger than EPUB for the same book?
MOBI files often contain two copies of the content — a MOBI-format version for older devices and a KF8 version for newer ones. This dual-format packaging roughly doubles the file size. EPUB stores content once in a compressed ZIP archive.
What makes .MOBI special
What is a MOBI file?
MOBI is an eBook format originally created by Mobipocket SA, later acquired by Amazon. Based on the Open eBook standard, MOBI files support DRM, bookmarks, annotations, and JavaScript. While Amazon has transitioned to KFX and AZW3 formats for newer Kindles, MOBI remains widely used for sideloading eBooks.
Continue reading — full technical deep dive
How to open MOBI files
- Kindle (devices and apps) — Native support
- Calibre (Windows, macOS, Linux) — Free, can convert
- FBReader (Windows, Linux, Android) — Free
- Sumatra PDF (Windows) — Lightweight reader
Technical specifications
| Property | Value |
|---|---|
| Base Format | Open eBook (OEB) |
| Compression | PalmDOC or HUFF/CDIC |
| DRM | Mobipocket DRM |
| Image Support | JPEG, GIF |
| Successor | AZW3 / KFX (Kindle Format) |
Common use cases
- Kindle sideloading: Manually adding books to Kindle devices.
- Legacy eBooks: Older digital book collections.
- Self-publishing: Distribution to Kindle readers.
- Format conversion: Source format for Calibre conversions.
.MOBI compared to alternatives
| Formats | Criteria | Winner |
|---|---|---|
| .MOBI vs .EPUB | Format openness and device support EPUB is an open standard (IDPF/W3C) supported by all major eBook platforms except Kindle. MOBI is proprietary and only natively supported on older Kindle devices and Calibre. | EPUB wins |
| .MOBI vs .AZW3 | Typography and layout capabilities AZW3 (KF8) supports HTML5, CSS3, embedded fonts, and SVG. MOBI is limited to basic HTML 3.2 subset with no custom fonts, fixed image positioning, and limited CSS. | AZW3 wins |
| .MOBI vs .PDF | Reflowable text MOBI text reflows to fit any screen size, adapting to phone, tablet, or e-ink displays. PDF has fixed page dimensions that require zooming and scrolling on small screens. | MOBI wins |
Technical reference
- MIME Type
application/x-mobipocket-ebook- Magic Bytes
42 4F 4F 4B 4D 4F 42 49BOOKMOBI at offset 60 in PDB header.- Developer
- Mobipocket SA (Amazon)
- Year Introduced
- 2000
- Open Standard
- No
BOOKMOBI at offset 60 in PDB header.
Binary Structure
MOBI files use the Palm Database (PDB) container format. The file starts with a 78-byte PDB header: 32 bytes for the database name (null-padded ASCII), 2-byte attributes, 2-byte version, 4-byte creation date (seconds since 1904-01-01), 4-byte modification date, and crucially, the type/creator fields at offsets 60-67. For MOBI files, the type field is 'BOOK' (42 4F 4F 4B) and the creator field is 'MOBI' (4D 4F 42 49), forming the identifying signature 'BOOKMOBI' at offset 60. After the PDB header, a record list follows with 8-byte entries (4-byte offset + 1-byte attributes + 3-byte unique ID) for each data record. Record 0 is the PalmDOC header (16 bytes): 2-byte compression type (1=none, 2=PalmDOC, 17480=HUFF/CDIC), 2 bytes unused, 4-byte uncompressed text length, 2-byte record count, 2-byte record size (typically 4096), and 2-byte encryption type (0=none, 1=old Mobipocket, 2=Mobipocket). Immediately after the PalmDOC header in Record 0 comes the MOBI header, starting with the ASCII string 'MOBI' (4D 4F 42 49) followed by header length, MOBI type (2=book, 3=PalmDoc-only), text encoding (65001=UTF-8, 1252=CP1252), unique ID, generator version, and offsets to EXTH header, first image record, and first content record. The EXTH header stores structured metadata as type-length-value records: author (100), publisher (101), description (103), ISBN (104), subject (105), and cover image index (201).
| Offset | Length | Field | Example | Description |
|---|---|---|---|---|
0x00 | 32 bytes | Database name | 4D 79 20 42 6F 6F 6B 00... | Null-padded ASCII name of the PDB database. Usually the book title truncated to 32 bytes. |
0x3C | 4 bytes | Type | 42 4F 4F 4B | PDB type field. ASCII 'BOOK' for Mobipocket/MOBI files. |
0x40 | 4 bytes | Creator | 4D 4F 42 49 | PDB creator field. ASCII 'MOBI' for Mobipocket/MOBI files. Combined with Type forms 'BOOKMOBI'. |
Record 0 + 0x00 | 2 bytes | Compression type | 00 02 | 1 = no compression. 2 = PalmDOC (LZ77). 17480 (0x4448) = HUFF/CDIC. |
Record 0 + 0x0C | 2 bytes | Encryption type | 00 00 | 0 = no DRM. 1 = old Mobipocket DRM. 2 = Mobipocket DRM. |
Record 0 + 0x10 | 4 bytes | MOBI identifier | 4D 4F 42 49 | ASCII 'MOBI'. Marks the start of the MOBI header within Record 0. |
Attack Vectors
- MOBI files can contain JavaScript (from Mobipocket Creator), though most modern Kindle firmware disables JS execution
- Malformed PDB record offsets can cause buffer overflows in older MOBI readers that lack bounds checking
- DRM-encrypted MOBI files may prompt users to install untrusted DRM-removal tools that bundle malware
Mitigation: FileDex does not open or parse MOBI files in the browser. MOBI is a reference-only page. Use Calibre for safe MOBI viewing and conversion — Calibre sanitizes content during import and does not execute embedded scripts.