Everything you need to know about MOBI
MOBI (.mobi) is Amazon Kindle's legacy ebook format - originally Mobipocket (2000), acquired by Amazon in 2005, and used as the default Kindle format until AZW3 replaced it in 2011. MOBI is essentially compressed HTML with proprietary navigation extensions, wrapped in Amazon's PalmDOC-derived container.
How it works under the hood
- PalmDOC heritage. Built on the PDB database format from PalmOS days. Each book is a database with metadata, an HTML body, and indexed records.
- Compression. MOBI uses custom LZ77 compression - decent for the era but worse than modern formats.
- Limited CSS. MOBI supports a subset of HTML 3.2 and minimal CSS. No web fonts, no advanced layout, no JavaScript.
- No variable fonts, no SVG. What you can do in EPUB 3 you mostly can't do in MOBI - it's a 2000-era format.
Where you'll actually use it
- Older Kindle devices (pre-2011)
- Calibre's MOBI export when sending books to legacy Kindles
- Backwards compatibility with old reading apps
- Public domain library archives (Project Gutenberg still ships MOBI)
How it compares to alternatives
MOBI vs EPUB: EPUB is the universal standard with much more capability. MOBI vs AZW3: AZW3 replaced MOBI in 2011 - better typography, embedded fonts, more features. Use AZW3 for current Kindles, EPUB for everything else.
Things that will trip you up
- Amazon stopped accepting MOBI uploads to KDP in 2022 - they want EPUB now and convert to AZW3 internally
- MOBI's TOC and navigation are fragile - test on actual Kindle hardware before distributing
- Image embedding is limited (typically 1MB cap per image, JPG/GIF only)
Test it yourself: Calibre (best MOBI tool), Amazon's free Kindle Previewer, MOBI Unpack for inspection. Test on real Kindle hardware where possible.