MOBI (Mobipocket and Kindle e-books)

MOBI is the e-book format of Mobipocket, which Amazon bought and turned into the Kindle format. A MOBI file is a Palm Database (PDB) file: a list of records that hold compressed HTML text, images and indexes, with metadata in a MOBI header and EXTH records. AZW is a MOBI file sold by Amazon, usually with Amazon DRM. AZW3, also called KF8 (Kindle Format 8), uses the same container but stores HTML5 and CSS3 content, embedded fonts and fixed layout.

MOBI is a legacy format. Amazon now takes EPUB for publishing (Kindle Direct Publishing) and for personal documents (Send to Kindle no longer accepts MOBI), and books bought on current Kindles are delivered in KFX, a different format. Kindle e-readers still open sideloaded .mobi and .azw3 files copied over USB, which is the main reason to create these files today.

When to use MOBI#

• Use AZW3 when you sideload your own books onto a Kindle e-reader over USB: it keeps CSS, fonts and the table of contents.
• Use a reader that supports MOBI to open old Mobipocket and Kindle files you already have.
• Avoid for publishing: Amazon KDP and Send to Kindle take EPUB and convert it themselves.
• Avoid the old MOBI 6 format (.mobi without KF8) for new books: it supports only basic HTML 3.2-like formatting, no CSS.

File structure#

A MOBI file has three layers: the Palm Database container, the PalmDOC header with the MOBI and EXTH headers in record 0, and the content records. All integers are big-endian.

PDB header (78 bytes)           name, dates, type "BOOK", creator "MOBI", record count
record list (8 bytes per rec)   offset of each record in the file
record 0                        PalmDOC header, MOBI header, EXTH header, full title
records 1..N                    compressed text (HTML), 4096 bytes uncompressed each
...                             index records (NCX/TOC, skeleton, fragments for KF8)
first image record ...          images (JPEG, GIF, PNG) as raw files, cover included
FLIS, FCIS, DATP, EOF           helper records

PDB header#

00000000  54 68 65 5F 42 6F 6F 6B 00 00 ...   "The_Book" (name, 32 bytes)
0000003C  42 4F 4F 4B 4D 4F 42 49             "BOOKMOBI" at offset 60
0000004C  01 2C                               300 records

Record 0: PalmDOC, MOBI and EXTH headers#

Record 0 starts with the 16-byte PalmDOC header:

The MOBI header follows at offset 16 of record 0. Important fields (offsets from the start of record 0):

The EXTH header (EXTH, length, record count) holds metadata as tagged records:

Text records#

Each text record holds up to 4096 bytes of uncompressed text, compressed with PalmDOC LZ77 (a simple byte-oriented scheme) or HUFF/CDIC (Huffman with a dictionary, used by Amazon’s tools for store books). Records can end with trailing entries (multibyte character overlap, indexing data) described by the “extra data flags” field of the MOBI header; decoders must strip them before decompressing.

• MOBI 6 text is one long HTML document with Mobipocket tags (<mbp:pagebreak/>), filepos attributes for links, and recindex attributes for images.
• KF8 text is HTML5 split into a skeleton and fragments (SKEL and FRAG index records) with an FDST record that separates HTML, CSS and SVG flows. Links and resources use kindle:pos:fid:... and kindle:embed:XXXX URLs.

MOBI, AZW, AZW3 and hybrid files#

DRM#

Books bought from Amazon (.azw, .azw3, KFX) are encrypted with a key tied to the Kindle device or app (encryption type 2 in the PalmDOC header). Only Amazon’s apps and devices open them. Readers and libraries that support MOBI, including SumatraPDF, Calibre and libmobi, open DRM-free files; libmobi can decrypt only with the device PID or serial number of the owner. Your own books created with Calibre or KindleGen have no DRM.

Converting#

• EPUB to AZW3 (for sideloading): ebook-convert book.epub book.azw3 with Calibre.
• MOBI/AZW3 to EPUB: ebook-convert book.azw3 book.epub, or mobitool -e book.mobi from libmobi, or KindleUnpack, which recovers the original KF8 source.
• For Kindle publishing: upload EPUB or DOCX to Kindle Direct Publishing, or use Amazon’s Kindle Previewer to check how a book looks on Kindle devices.

Specifications and documentation#

• MOBI format on the MobileRead wiki: the most complete description of the PDB, MOBI and EXTH headers
• KF8 on the MobileRead wiki: Kindle Format 8 structure
• PDB format on the MobileRead wiki: Palm Database container and PalmDOC compression
• Library of Congress: Mobipocket format description
• Calibre MOBI source code: reader and writer for MOBI 6 and KF8, the de facto reference
• Amazon: Kindle Publishing Guidelines: what Kindle supports in KF8
• Mobipocket on Wikipedia

Software that opens MOBI files#

Libraries#

Open-source libraries that read and write MOBI files. See all MOBI libraries for a comparison.

Related formats#

• EPUB: the open e-book standard; Amazon converts EPUB to its own formats
• FB2: XML e-book format
• PDF: Print Replica (.azw4) wraps a PDF
• PDB: Palm Database, the container of MOBI; PalmDOC e-books use it directly