MOBI File (.mobi): What It Is, How It Works + Sample Files

The basics

What is a MOBI file?

MOBI is the e-book format of Mobipocket, a French company founded in 2000, whose format grew out of the database files of Palm handhelds. Amazon bought Mobipocket in 2005 and built the first Kindles on its format, so for years MOBI was the way to get a book onto a Kindle.

Inside, a MOBI file holds the book as HTML, compressed in small pieces, plus images, a table of contents and metadata. It reflows like an EPUB: the reading app decides the line breaks and pages. Unlike EPUB, it's a binary format owned by one company and was never fully published.

It's a legacy format. Amazon replaced it with AZW3 (KF8) in 2011 and has since stopped accepting MOBI for publishing and for Send to Kindle. Most new MOBI files today come from converters such as Calibre.
It's the base of Amazon's formats. AZW is a MOBI file with Amazon's extension and usually its DRM, and AZW3 keeps the same container with new content inside.
.prc is the same format. The name comes from Palm resource files; old Mobipocket books often used it.

Anatomy

What's inside a MOBI file

A MOBI file is a Palm database: a 78-byte header, a list of records, then the records themselves. Here is sample-mobi6.mobi taken apart, from the header to the end-of-file record, with the start of record 0, where the book's headers and metadata live. Offsets are in hex in the dumps and in decimal in the record list.

sample-mobi6.mobi (header, record list, record 0)

sample-mobi6.mobi: 123,321 bytes, 11 records

Palm database header (bytes 0-77)
00000000  41 5F 53 6D 61 6C 6C 5F …   A_Small_Guide_to_Paper_Boats
00000024  6A E0 FB 87 6A E0 FB 87     created, modified
0000003C  42 4F 4F 4B 4D 4F 42 49     BOOKMOBI
0000004C  00 0B                       11 records

Record list (offset, size, what)
rec  0  @168        9,072  PalmDOC, MOBI and EXTH headers
rec  1  @9,240      1,788  the text: 3,971 bytes of HTML, compressed
rec  2  @11,028       240  INDX  table of contents index
rec  3  @11,268       236  INDX  index entries
rec  4  @11,504       100  CNCX  chapter titles for the index
rec  5  @11,604    81,665  JPEG  cover
rec  6  @93,269    10,279  JPEG  thumbnail
rec  7  @103,548   19,689  JPEG  folding diagram
rec  8  @123,237       36  FLIS
rec  9  @123,273       44  FCIS
rec 10  @123,317        4  E9 8E 0D 0A  end of file

Record 0
000000A8  00 02 00 00 00 00 0F 83 00 01 10 00 00 00 00 00
000000B8  4D 4F 42 49 00 00 00 E8 00 00 00 02 00 00 FD E9
000000C8  8D 16 F7 4A 00 00 00 06 …
000001A0  45 58 54 48 00 00 02 5B 00 00 00 16
           524 language       en
           503 updated title  A Small Guide to Paper Boats
           100 author         Ada Example
           103 description    A free sample e-book from 99helpers.com/file-types/mobi…
           501 cdetype        EBOK
           201 cover offset   0
          … 22 records
          then the full title and 8,192 bytes of zero padding

1A_Small_Guide_to_Paper_Boats
The database name
The first 32 bytes are a Palm database name, padded with zeros. Converters put a short form of the title here, with spaces replaced; the real title is stored later.
26A E0 FB 87 6A E0 FB 87
Timestamps
Creation and modification times in seconds. Calibre writes Unix time, while the original Palm format counted from 1904, so some tools show dates decades off. Nothing depends on them.
342 4F 4F 4B 4D 4F 42 49
BOOKMOBI
The type BOOK and creator MOBI at bytes 60 to 67 identify the file. This is the check file-type detectors use; AZW and AZW3 files have the same bytes.
4Record list
The record list
Eight bytes per record: where it starts, a flags byte and a unique ID. A reader can load one record at a time instead of the whole file.
500 02 00 00 00 00 0F 83 00 01 10 00
The PalmDOC header
The first 16 bytes of record 0: compression 2 (PalmDOC; 1 means none, 17480 means HUFF/CDIC), 3,971 bytes of text in 1 record of at most 4,096 bytes, and encryption 0. A book with Amazon's DRM has a non-zero encryption value here.
64D 4F 42 49 00 00 00 E8
The MOBI header
MOBI and the header length (232 bytes), then the book type (2, a book), the text encoding (65001, UTF-8; older files use 1252, Windows Latin-1), a unique ID and the file version: 6 for a classic MOBI, 8 for KF8.
745 58 54 48
EXTH metadata
A list of numbered records: 100 is the author, 103 the description, 524 the language, 201 which image is the cover, 501 the document type (EBOK for a book). This is what ebook-meta and Kindle libraries read.
8rec 1
The text records
The whole book is one stream of HTML, cut into 4,096-byte pieces that are compressed one at a time. Our book's text fits in one record.
9INDX / CNCX
The index
The table of contents a Kindle shows in its Go To menu, stored as index records with the chapter titles in a separate string table.
10JPEG
Images
Each image is its own record, referred to by its number. By default Calibre converts every image to JPEG for MOBI output.
11E9 8E 0D 0A
End of file
A fixed 4-byte record that ends the book. FLIS and FCIS before it are short fixed-format records that Calibre writes too.

Under the hood

How a MOBI file stores a book

Strip away the container and a classic MOBI (version 6) is one long HTML document in an old dialect. Decompress the text of sample-mobi6.mobi and you find this, not the XHTML and CSS of the EPUB it was made from:

sample-mobi6.mobi, record 1 decompressed (excerpt)

<p height="2em" width="0pt"><font size="6"><b>Contents</b></font></p>
<ol width="0pt"><li value="1"><a filepos=0000000462>Chapter 1. Choosing the paper</a></li> …
<mbp:pagebreak/>
<p height="1em" width="0pt" align="center"><img recindex="00003" width="600" height="260"></img></p>

No stylesheets. MOBI 6 has no CSS. Converters turn styles into old HTML attributes such as <font size> and align, so fonts, margins and colours come out simpler than in the source.
Links by byte position. filepos=0000000462 points to a byte offset in the decompressed text, not to an ID. Change the text and every link after it must be recalculated.
Images by record number. recindex refers to the image records, counted from the first image.
Page breaks are a Mobipocket tag, <mbp:pagebreak/>.
Compression per record. The text is cut into 4,096-byte pieces, each compressed with PalmDOC compression, a simple LZ77 variant, so a reader can open page 200 without decompressing pages 1 to 199. Some files use the stronger HUFF/CDIC compression instead.

A joint MOBI/KF8 file

From 2011, Amazon's KindleGen 2 wrote both versions into one .mobi: the old MOBI 6 book first, then a BOUNDARY record, then the same book as KF8 with HTML5 and CSS. EXTH record 121 in the first part says where the KF8 part starts. One file could then serve both older Kindles and KF8 devices. sample-book.mobi is built this way.

Timeline

From Mobipocket to Kindle, and out again

2000: Mobipocket SA is founded in France and introduces its format, based on the Palm database format.
2005: Amazon buys Mobipocket.
2007: the first Kindle reads Amazon's AZW books, which are MOBI files with Amazon's DRM, and unprotected MOBI files.
2011: Amazon announces Kindle Format 8 (KF8), with HTML5 and CSS3, replacing MOBI 7 (as Amazon called its MOBI version); the Kindle Fire is the first device to support it. KindleGen 2 starts writing joint MOBI/KF8 files. Mobipocket's own shop stops selling e-books late that year.
1 August 2021: Kindle Direct Publishing (KDP) stops accepting MOBI files for new reflowable books and updates.
November to 20 December 2023: Amazon winds down MOBI support in Send to Kindle, its service for sending your own documents to a Kindle, ending it by 20 December 2023. The change covered .mobi, .azw and .prc files.
18 March 2025: KDP stops accepting MOBI for fixed-layout books as well.

Books and documents already in a Kindle library weren't affected by the Send to Kindle change, and books already published from MOBI files stay on sale. What changed is that you can't add new ones this way: Amazon now points to EPUB for both.

Comparison

MOBI vs KF8 (AZW3) vs EPUB

	MOBI (version 6)	KF8 / AZW3	EPUB 3
Introduced	2000	2011	2011 (EPUB 2: 2007)
Container	Palm database (`BOOKMOBI`)	Palm database (`BOOKMOBI`)	ZIP
Content	Old HTML dialect, no CSS	HTML5 and CSS3, stored in pieces	XHTML (HTML5), CSS, SVG
Fixed layout	No	Yes	Yes
Embedded fonts	No	Yes	Yes
Specification	Not published	Not published	Open, W3C Recommendation
Accepted by Send to Kindle	No (since December 2023)	No	Yes
Best use today	Old devices and apps that read nothing newer	Kindle e-readers, copied by USB	Everything else: publishing, libraries, most reading apps

If you have a choice, keep books as EPUB and convert only when a device needs something else. Converting MOBI to EPUB works well for plain novels but can't bring back styling that the MOBI never had.

Troubleshooting

Why won't my MOBI file open, send or look right?

Send to Kindle rejects it. Amazon's service no longer takes MOBI files. Convert the book to EPUB, for example with Calibre, and send that.
KDP rejects it. KDP no longer takes MOBI uploads. Upload the EPUB, Word or KPF file you made it from.
It won't open in another app. The book has DRM (common for files that came from Amazon, often as .azw), or the file is really AZW3 or KFX with a .mobi name. Check bytes 60 to 67 for BOOKMOBI, and the encryption value in record 0.
The formatting is plain. That's MOBI 6: no CSS, so tables, fonts, margins and colours are simplified. A joint file or an AZW3 keeps more of the original design on devices that read KF8.
The cover is missing. The cover is found through EXTH record 201, an offset from the first image record. Converters that leave it out or point it at the wrong image produce a book with no thumbnail.
Accented letters are garbled. Some MOBI files are in Windows-1252 (code page 1252) rather than UTF-8. The MOBI header records which one; tools that ignore it show mojibake.

Protection

DRM in MOBI files

Mobipocket defined its own encryption, and Amazon's DRM for Kindle books in this format uses the same encryption field. A protected book has a non-zero encryption type in its PalmDOC header, and its text can only be decrypted with a key tied to a Kindle device or app registered to the buyer's Amazon account. The headers and metadata stay readable, which is why a library can list a protected book it can't open.

Calibre and other tools refuse to open or convert books with DRM. Removing DRM breaks the shop's terms and is illegal in many countries. Files you make yourself, and all our samples, have no DRM.

How to

How to open, edit and convert a MOBI file

Read it: Calibre's e-book viewer opens DRM-free MOBI files on Windows, macOS and Linux. Browsers don't display MOBI, so a link to one downloads it.
Preview it as a Kindle would: Amazon's free Kindle Previewer for Windows and macOS shows how a book displays on Kindle devices and apps.
Check its metadata: ebook-meta book.mobi prints the title, author, description and more from the EXTH records.
Convert it to EPUB: ebook-convert book.mobi book.epub with Calibre. Then you can send it to a Kindle, read it anywhere and edit it in Sigil or Calibre's editor.
Make one: ebook-convert book.epub book.mobi writes MOBI 6. Add --mobi-file-type both for a joint MOBI/KF8 file or new for KF8 only (for that, the AZW3 output is the usual choice).
Look inside: calibre-debug --inspect-mobi book.mobi dumps every header and record, and the decompressed text, into a folder.

Free downloads

Sample MOBI files to download

Free, DRM-free sample MOBI files for testing e-book readers, converters, metadata extraction and file-type detection. All three hold the same short book, "A Small Guide to Paper Boats" by the fictional Ada Example, made with Calibre 7.6 from an EPUB we wrote, and all three convert back to EPUB with every chapter and image intact.

sample-book.mobi · 134.5 KBDownload

Browsers can't show this kind of file in a page. Download it to open it in the app it belongs to.

What's inside sample-book.mobi

Browsers can't display MOBI, so there's no live preview here. Open sample-book.mobi in Calibre's viewer to read the book, or run calibre-debug --inspect-mobi on it to see these parts.

Two books in one file
Records 0 to 9 are a MOBI 6 book, record 10 is the 8-byte BOUNDARY marker, and records 11 to 26 are the same book as KF8 (file version 8). The KF8 part has no images of its own: it uses those of the first part.
Metadata
EXTH records for the title, the author Ada Example, publisher, subject, publication date, language and a description that names this page.
A cover and a thumbnail
A 600 × 900 cover drawn for this sample, plus the smaller thumbnail Kindle libraries use, both found through EXTH records 201 and 202.
A table of contents
A Contents page with links to the three chapters, and an index (NCX) for a Kindle's Go To menu.
Chapter 2: an image, a list and a table
A folding diagram with alt text, a numbered list and a table with a caption. The table's cell borders come from CSS, so only the KF8 half has them.
Compressed text
The text records use PalmDOC compression, as nearly all MOBI files do. sample-uncompressed.mobi is the uncompressed counterpart.

All sample MOBI files

Joint MOBI/KF8 book
sample-book.mobi · 134.5 KB (137,779 bytes)
A three-chapter book stored twice, as MOBI 6 and as KF8, the way KindleGen 2 built Kindle books from 2011. Cover, table of contents, image, list and table.
- Testing readers that pick the KF8 part when they can
- Checking parsers handle the BOUNDARY record
Download
Classic MOBI 6 book
sample-mobi6.mobi · 120.4 KB (123,321 bytes)
The same book as MOBI 6 only: one compressed text record, an index and three JPEG images. It's the file taken apart in the anatomy above.
- Old readers and apps that only know Mobipocket
- Learning the format's structure
Download
Uncompressed MOBI 6
sample-uncompressed.mobi · 122.6 KB (125,509 bytes)
MOBI 6 with compression type 1 (none): the HTML of record 1 is readable in a hex editor.
- Testing that parsers read the compression field instead of assuming PalmDOC
- Seeing the MOBI 6 markup without decompressing it
Download

Ways to test with them

File type detection: all three have BOOKMOBI at byte 60, like AZW and AZW3 files. A detector that needs to tell them apart has to read the MOBI header's file version or look for a KF8 boundary.
Metadata extraction: run ebook-meta or your own parser and expect the title "A Small Guide to Paper Boats", the author Ada Example and a description mentioning 99helpers.com/file-types/mobi.
Converters: convert each sample to EPUB and check that you get three chapters, the cover and the diagram.
Joint files: open sample-book.mobi in a KF8-capable reader and in a MOBI-only one. The table in Chapter 2 has cell borders only in the KF8 half, which is a quick way to see which half was used.

Free to use for testing, teaching and demos, commercially or not, with no attribution needed. A link back to this page is always appreciated.

Put it online

Have a MOBI file? Give it a link.

Drop a MOBI file on 99helpers and it gets its own link to share. Browsers can't display MOBI files, so visitors download it and open it in an e-book app.

Put a MOBI file online

Questions

MOBI files, frequently asked

What is a MOBI file?

An e-book in the Mobipocket format, which Amazon bought in 2005 and used for the first Kindles. It holds compressed HTML, images and metadata in a Palm database file.

How do I open a MOBI file?

Use Calibre's e-book viewer on Windows, macOS or Linux, or Amazon's Kindle Previewer. Browsers can't open MOBI files. If the file has DRM, only the Kindle app or device it was bought for can open it.

Can I still send a MOBI file to my Kindle?

Not with Send to Kindle: Amazon stopped accepting MOBI files there by 20 December 2023. Convert the book to EPUB with Calibre and send the EPUB instead. MOBI files already in your Kindle library still work.

What's the difference between MOBI and AZW3?

Both use the same container and start with BOOKMOBI. MOBI 6 holds an old HTML dialect without CSS; AZW3 (KF8) holds HTML5 and CSS3 and supports embedded fonts and fixed layout.

Should I use MOBI or EPUB?

EPUB. It's an open standard, most reading apps read it, and Amazon now asks for EPUB both for Send to Kindle and for publishing on KDP.

How do I convert MOBI to EPUB?

Add the book to Calibre and use Convert books, or run ebook-convert book.mobi book.epub. It only works for files without DRM.

Does Amazon KDP accept MOBI files?

No. KDP stopped accepting MOBI for reflowable books on 1 August 2021 and for fixed-layout books on 18 March 2025. It accepts EPUB, Word and KPF files instead.

What is a .prc file?

The same Mobipocket format under an older extension, from the Palm resource files the format grew out of. Rename it to .mobi and MOBI readers open it.

Related formats

Other file types explained

AZW3 file AZW file EPUB file FB2 file PDF file HTML file All file types

Keep going

Host a MOBI file

Put an e-book online at its own address.

Share a MOBI file

Send a MOBI book as a link instead of an attachment.

Sources

Last reviewed 27 October 2026.