What is the MOBI file format?
.mobi files store digital eBook content in the Mobipocket binary eBook format, built on the .pdb (Palm Database) file structure. The .mobi extension supports standard HTML markup for text content and is widely used for eBook distribution, particularly on Amazon Kindle devices and smartphones.
The .mobi format includes optional DRM (Digital Rights Management) copyright protection features, useful for preventing unauthorized copying or viewing of the eBook content it contains. DRM-protected .mobi files require a licensed reader application to open.
Originally developed by Mobipocket SA and acquired by Amazon in 2005, .mobi became the foundation for Amazon's early Kindle ecosystem. Amazon later replaced it with .azw and the more capable .azw3 (KF8) format, retiring .mobi support from its Send to Kindle service in 2022.
.mobi files can still be read by many third-party applications. Calibre remains the leading tool for opening, editing, and converting .mobi to formats such as .epub.
Security & safety
RISK: LOWMOBI is e-book data and safe to read. The real friction is DRM, not malware: AZW/MOBI books bought from Amazon are account-locked, so generic apps refuse them and conversion is blocked - a rights restriction, not a virus. Removing DRM violates Amazon's terms and may be unlawful where you live. The practical risk is pirated-e-book download sites (bundled adware/fake downloads). Treat DRM-free MOBI files as ordinary documents.
Format details
in a nutshellFULL NAMEMobipocket eBook
DEVELOPERMobipocket SA (acquired by Amazon, 2005); format retired by Amazonsince 2000 (Mobipocket); retired from Send to Kindle by Amazon in 2022
CATEGORYeBook Files
MIME TYPEapplication/x-mobipocket-ebook
TYPEPalm Database (PDB)-based e-book container wrapping compressed HTML; may carry DRM
MAGIC BYTES · FILE SIGNATURE
OFFSET
3c3d3e3f40414243
HEX
424F4F4B4D4F4249
ASCII
BOOKMOBI
MOBI is a Palm Database (PDB) file. There is no fixed magic at offset 0 (the first 60 bytes are the variable book name). At offset 60 are the 8 ASCII bytes "BOOKMOBI" (42 4F 4F 4B 4D 4F 42 49) = type "BOOK" + creator "MOBI". The same PDB lineage underlies AZW and AZW3.
Programs that open MOBI files
Windows3 apps
Calibre
Open-source Add the .mobi to the library (drag & drop) to read in the viewer, and to convert it to EPUB/PDF. DRM-free files only. 
Kindle for PC
Free Reads MOBI/AZW books delivered to your Amazon account; the official reader for DRM-protected purchases. 
macOS3 apps
Linux3 apps
Calibre
Open-source Primary option on Linux: add the .mobi, read in the built-in viewer, convert to EPUB/PDF. 
Android3 apps
Amazon Kindle (app)
Free Read account books; sideload a DRM-free MOBI via the app's import or Documents folder.
iOS1 app
Technical details
deep spec| Container base | Palm Database (PDB) file structure; each .mobi file is a valid PDB file |
| Type/creator identifier | "BOOK" (type) + "MOBI" (creator) - 8-byte ASCII string "BOOKMOBI" at byte offset 60 |
| MIME type | application/x-mobipocket-ebook |
| Text compression | PalmDOC LZ77-based compression or Huffman/CDIC (HuffCDIC) encoding for later MOBI versions |
| DRM support | Optional; Mobipocket DRM and an Amazon-specific variant for Kindle-purchased titles |
| Image embedding | JPEG and GIF images stored as separate PDB records; limited PNG support in later MOBI versions |
| Metadata storage | EXTH header block with typed records for title, author, publisher, language, and ISBN |
| Table of contents | Encoded via HTML anchor links with an optional EXTH NCX reference record |
| Layout model | Reflowable text only; no fixed-layout or page-size locking support |
| JavaScript support | Not supported; no scripting layer in the format specification |
| Font embedding | Not supported; text rendering uses fonts available on the reading device or application |
| Record structure | Book text split across chained 65,536-byte PalmDOC records; images and metadata in separate records |
| Format lineage | Direct ancestor of .azw (Kindle Mobi7) and influenced the KF8 (.azw3) container design |
| Developer / steward | Mobipocket SA (2000-2005); Amazon.com (2005-2022) |
| Service retirement | Removed from Amazon's Send to Kindle service in 2022; superseded by the KFX format |
| Released | 2000 (Mobipocket); retired from Send to Kindle by Amazon in 2022 |
| Specification | wiki.mobileread.com |
MOBI conversions
Possible
.MOBI .PDFGUIDE
Yes - Calibre converts a DRM-free MOBI file to PDF in a few clicks on any desktop OS.
How to convert
.MOBI .EPUBGUIDE
Yes, MOBI converts to EPUB - Calibre does it in seconds for DRM-free files, and it is the recommended path…
How to convert
.MOBI .TXTGUIDE
Yes, MOBI converts to TXT -- the fastest way is Calibre's command-line tool: ebook-convert book.mobi book.txt
How to convert
.MOBI .DOCXGUIDE
Yes, a DRM-free Mobipocket e-book can be converted to DOCX - Calibre does it in a few clicks on any desktop…
How to convert
.MOBI .FB2GUIDE
Yes, MOBI converts to FB2 - Calibre does it in a few clicks on any desktop OS.
How to convert
.MOBI .AZW3GUIDE
Yes - Calibre (free, all platforms) converts a DRM-free MOBI to AZW3 in a few clicks, and it is the right…
How to convert
Not possible
Community Q&A
asked by users
N
Frequently asked questions
How do I open a MOBI file?
Use Calibre on Windows/macOS/Linux to read and convert DRM-free MOBI, or a reader like FBReader/Cool Reader. Books bought from Amazon (DRM) open in the official Kindle app on your account.
How do I read a MOBI on a Kindle?
Amazon dropped MOBI from Send to Kindle in 2022, so you can't send a loose .mobi anymore. Convert it to EPUB in Calibre and send the EPUB, or sideload an AZW3 over USB. Books already bought in your account stay readable.
How do I convert MOBI to EPUB?
Open the file in Calibre, click Convert, choose EPUB. This works on DRM-free MOBI. Amazon-purchased books are DRM-protected and can't be converted without removing the protection (against Amazon's terms).
What's the difference between MOBI and EPUB?
MOBI is Amazon's old, now-retired Mobipocket format; EPUB is the open industry standard supported almost everywhere (including, since 2022, Send to Kindle). For new files always prefer EPUB.
Why won't my MOBI file convert in Calibre?
Almost always DRM: AZW/MOBI books bought from Amazon are copy-protected, so Calibre refuses them. DRM-free MOBI files (your own, or free books) convert without issue.
MOBI vs AZW vs AZW3 - what's the difference?
All share the Palm-DB lineage. MOBI is the original Mobipocket format; AZW is MOBI plus early Kindle DRM; AZW3 (KF8, 2011) is the newer, better-styled Kindle format. MOBI is the oldest and now retired.
References
1Library of Congress - Mobipocket / MOBI formatwww.loc.gov
2MobileRead Wiki - MOBI formatwiki.mobileread.com
