EPUB and MOBI are both reflowable ebook formats — they adapt their text layout to fit the screen they're displayed on. The fundamental difference is ecosystem: EPUB is the open international standard used by every ebook platform except Amazon; MOBI (and its successor EPUB-based Kindle formats) is Amazon's proprietary format family.
MOBI originated from Mobipocket, a French company Amazon acquired in 2005. Amazon extended Mobipocket's .mobi format into the Kindle ecosystem, adding DRM (Digital Rights Management) and eventually evolving it into AZW, AZW3, and KFX formats. For over a decade, if you wanted to read an ebook on a Kindle, you needed a Kindle-compatible format.
In 2022, Amazon started accepting EPUB for Kindle Direct Publishing (KDP) submissions and 'Send to Kindle' transfers. They convert submitted EPUB to their internal format. This effectively made EPUB the better starting point for Kindle content too. MOBI's primary relevance today is for older content and older Kindle devices.
What EPUB and MOBI Are
EPUB: The Open Standard
EPUB (Electronic Publication) is maintained by the W3C (World Wide Web Consortium) and the IDPF (International Digital Publishing Forum). The current version is EPUB 3, which uses HTML5, CSS3, SVG, MathML, and JavaScript as its building blocks. An EPUB file is a ZIP archive containing these web technologies plus OPF (Open Packaging Format) metadata and a navigation document.
EPUB 3 supports embedded audio and video (for enhanced ebooks), accessible markup, fixed-layout profiles (for picture books and comics), and internationalization including right-to-left text. DRM in EPUB is applied by the distribution platform — the same EPUB file can be DRM-free (Smashwords, direct downloads) or encrypted (Adobe DRM via OverDrive, Kobo, Sony Reader).
MOBI: Amazon's Kindle Legacy Format
MOBI (Mobipocket) uses a proprietary binary format. The file contains HTML-like content, images, and metadata packed into a custom binary structure (Palm Database format with HTML extensions). MOBI supports basic text formatting, embedded images, basic tables, and NCX navigation (the same standard as older EPUB). It does not support JavaScript, audio, video, or the full CSS3 features of EPUB 3.
Amazon's Kindle format evolution: MOBI (original) → AZW (MOBI with Amazon DRM) → AZW3/KF8 (EPUB-like, better CSS support) → KFX (current proprietary format used internally). When Amazon says 'Kindle format,' they mean this entire family. Modern Kindles use KFX internally; MOBI files are supported for backward compatibility but converted on sideload or download.
In 2022, Amazon deprecated MOBI for KDP submissions, instead accepting EPUB and DOCX. However, MOBI files can still be sideloaded to Kindles via USB and will be rendered (on newer devices, converted to KFX first).
Technical Comparison: EPUB vs MOBI
| Feature | EPUB 3 | MOBI |
|---|---|---|
| Standard type | Open international (W3C/IDPF) | Proprietary (Amazon/Mobipocket) |
| File structure | ZIP of HTML/CSS/images | Binary (Palm Database + HTML) |
| CSS support | Full CSS3 | Limited (no CSS3 advanced features) |
| JavaScript | Yes (EPUB 3) | No |
| Audio/video | Yes (EPUB 3) | No |
| MathML | Yes | No |
| Fixed layout | Yes (EPUB 3 FXL) | Limited |
| SVG images | Yes | Limited |
| Typography | Web fonts (WOFF2) | Limited font embedding |
| DRM | Adobe DRM, Readium LCP | Amazon DRM (KFX for purchased) |
| Table of contents | EPUB navigation document | NCX + MOBI-specific TOC |
| Max file size | Effectively unlimited | ~650 MB practical limit |
Device and Platform Support
| Platform | EPUB | MOBI |
|---|---|---|
| Kindle (current devices) | Accepted by KDP; converts to KFX | Supported (converted to KFX) |
| Kindle (older, pre-2022) | Not directly; use KDP or Calibre conversion | Native format |
| Apple Books | Native (preferred format) | Not supported |
| Kobo | Native (full EPUB 3 support) | Not supported |
| Google Play Books | Native | Not supported |
| Nook | Native | Not supported |
| OverDrive / Libby (libraries) | Native | Not supported |
| Calibre (desktop) | Full read/convert | Full read/convert |
| Readium | Native | No |
File Size Comparison
For most ebooks, EPUB and MOBI files of equivalent content are similar in size. MOBI sometimes bundles multiple format variations (a combined MOBI file may contain both an older MOBI record and a KF8/AZW3 record), making it larger.
| Content Type | EPUB | MOBI |
|---|---|---|
| Novel (text-heavy, 300 pages) | 200–500 KB | 300–700 KB |
| Nonfiction with images (300 pages) | 5–20 MB | 6–25 MB |
| Technical book with code samples | 2–8 MB | 3–12 MB |
MOBI files are often slightly larger than equivalent EPUB because of their binary structure and, in some tools' output, embedding both MOBI and KF8 format data in a single file (the 'combo MOBI').
Use Case Decision Matrix
| Use Case | Best Format | Why |
|---|---|---|
| Publishing on Kindle Direct (KDP) | EPUB | Amazon's current preferred submission format; better conversion quality |
| Publishing on Apple Books | EPUB | Required format |
| Publishing on Kobo, Nook, Google Play | EPUB | Required format |
| Library lending (OverDrive) | EPUB | Standard format; MOBI not supported |
| Sideloading to old Kindle (pre-2018) | MOBI | Direct USB sideload without conversion needed |
| Personal ebook collection (Calibre) | Either (convert with Calibre) | Calibre handles both; EPUB as master copy is best practice |
| DRM-free ebook distribution | EPUB | Open standard; readable on any device with any reader app |
When to Use EPUB vs MOBI
Use EPUB When...
- Publishing anywhere except directly to old Kindles — Every major ebook platform except Amazon's oldest devices uses EPUB natively
- Distributing DRM-free ebooks — Smashwords, Gumroad, direct downloads: EPUB works on every app
- You want the best feature support — CSS3, web fonts, audio/video, MathML — EPUB 3 has all of it; MOBI doesn't
- Creating accessible ebooks — EPUB 3's semantic HTML maps perfectly to screen reader accessibility standards
- Building a library for multiple platforms — One EPUB file can be converted to any other format; it's the master format
Use MOBI When...
- Sideloading to older Kindle devices — Kindles manufactured before 2022 can sideload MOBI via USB more reliably than EPUB
- You have existing MOBI content to distribute — MOBI files work fine for Kindle email delivery and USB transfer
- Specific older Kindle apps or devices require it — Some older Kindle Fire models and early Kindle Paperwhite generations handle MOBI natively without conversion
The practical advice: In 2026, create your ebook in EPUB. If you need a Kindle version, Calibre converts EPUB to MOBI/AZW3 reliably. Amazon KDP accepts EPUB directly. Keep EPUB as your master format.
Convert EPUB to MOBI (or MOBI to EPUB) with ChangeThisFile
ChangeThisFile supports EPUB ↔ MOBI conversion via /epub-to-mobi and /mobi-to-epub. Conversions use Calibre's ebook-convert tool, which is the gold standard for ebook format conversion.
curl -X POST https://changethisfile.com/v1/convert \
-H "Authorization: Bearer YOUR_API_KEY" \
-F "file=@ebook.epub" \
-F "target=mobi" \
-o ebook.mobi690 routes supported. Free for 1,000 conversions/month.