CONVERT MOBI → FB2 (MOBI to FB2)
Fast, secure MOBI to FB2 conversion. No registration required.
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Ghostscript + QPDFHOW TO CONVERT MOBI → FB2
Upload the MOBI
Drop the ebook file into the uploader. We detect the format and extract metadata automatically.
Convert through Calibre
Calibre parses the MOBI structure, reflows content and writes a FB2 with the appropriate CSS profile for the target readers.
Download the FB2
Grab the converted ebook; both files auto-delete within two hours of the job finishing.
Situation. MOBI is Amazon's legacy Mobipocket ebook format, the older Kindle delivery format. Solution: a FB2, produced below. Our MOBI to FB2 converter uses the same Calibre engine that powers the bulk of the ebook world. Authors use it, bookstores use it, pirates (unfortunately) use it — it is the de-facto standard for ebook format conversion and we just expose it over a simple browser upload. A quick refresher — MOBI is Amazon's legacy Mobipocket ebook format, the older Kindle delivery format. By contrast, FB2 is FictionBook, a popular XML-based open ebook format in Russian-speaking markets.
Why convert MOBI to FB2
FB2 works on your specific reader where MOBI does not. Ebook formats are tied to ecosystems — FB2 is Amazon-native, FB2 is IDPF-standard and opens everywhere, pick the one that matches your device and convert the rest.
MOBI vs FB2 — Technical specifications
Side-by-side comparison of the technical details.
MOBI
- MIME type
- application/x-mobipocket-ebook
- Extension
- .mobi, .prc (PalmDOC variant)
- Container
- Palm Database (PDB)
- Markup
- Compressed HTML subset
- Successor
- .azw, .azw3 (Kindle-specific)
- MIME Type
- application/x-mobipocket-ebook
- Category
- eBooks
- Full Name
- Mobipocket eBook
- Conversion Engine
- CALIBRE
FB2
- MIME type
- application/x-fictionbook+xml
- Extension
- .fb2
- Container
- Single XML file (optionally zipped)
- MIME Type
- application/x-fictionbook+xml
- Category
- eBooks
- Full Name
- FictionBook
- Conversion Engine
- CALIBRE
- Extensions
- .fb2, .fb2.zip
- Standard
- FictionBook community spec (maintained on GitHub)
- Encoding
- UTF-8 (required)
| Specification | MOBI | FB2 |
|---|---|---|
| MIME type | application/x-mobipocket-ebook | application/x-fictionbook+xml |
| Extension | .mobi, .prc (PalmDOC variant) | .fb2 |
| Container | Palm Database (PDB) | Single XML file (optionally zipped) |
| Markup | Compressed HTML subset | — |
| Successor | .azw, .azw3 (Kindle-specific) | — |
| MIME Type | application/x-mobipocket-ebook | application/x-fictionbook+xml |
| Category | eBooks | eBooks |
| Full Name | Mobipocket eBook | FictionBook |
| Conversion Engine | CALIBRE | CALIBRE |
| Extensions | — | .fb2, .fb2.zip |
| Standard | — | FictionBook community spec (maintained on GitHub) |
| Encoding | — | UTF-8 (required) |
MOBI vs FB2 — Strengths and limitations
What each format does best, and where it falls short.
MOBI Strengths
- Universal Kindle support on every device ever released.
- Very small file sizes for text-heavy books.
- Mature tooling via Calibre and Amazon's KindleGen.
- Simple container structure — easy to parse.
Limitations
- Deprecated by Amazon for new uploads since 2022.
- Poor support for rich typography (drop caps, ligatures, fixed layout).
- No embedded fonts in the base format.
FB2 Strengths
- Pure XML — trivial to parse, search, and transform.
- Single-file ebooks with inline images.
- Excellent on low-powered e-readers.
- De-facto standard in Russian-language ebook ecosystem.
- Simple structure makes conversion to any other format straightforward.
Limitations
- No styling, no custom fonts, no fixed layouts.
- Minimal Western-language tooling.
- Kindle and Apple Books do not support FB2 natively.
Quality & Compatibility
Text content is preserved losslessly — every word of the MOBI ends up in the FB2. Formatting richness depends on what the FB2 container supports; heavy typography that works in MOBI may degrade gracefully in FB2, never lost entirely.
Tips for Best Results
- Set author and title metadata in Advanced before conversion; fixing it after sideloading to a reader is far more work.
- If chapter navigation feels off in the FB2, the source MOBI probably had a weak ToC — Calibre respects what it finds, it cannot invent structure that was not there.
- For fixed-layout MOBI (art books, technical manuals with diagrams) consider whether a reflowable FB2 actually makes sense before converting.
Frequently Asked Questions
Yes. The free tier accepts files up to 25 MB without registration, email capture or watermarks; paid plans go up to 2 GB. Paid plans raise the size cap, enable batch conversions and provide a REST API for automation, but nothing on the free tier is quality-limited — the output is exactly the same as on any paid plan.
Yes, provided the MOBI itself has a well-formed ToC. Calibre reads the navigation structure and writes an equivalent ToC into the FB2. If the source lacks a ToC we can generate one from heading levels in Advanced → structure detection.
Uploads run over HTTPS, files are processed in isolated containers, and both the source MOBI and the FB2 output are auto-deleted within two hours. No account is required, file contents are never logged, and KaijuConverter does not use uploads for AI training. The paid plan adds a signable data-processing agreement for regulated workflows.
No. KaijuConverter does not strip digital rights management. DRM-free MOBI files — anything you authored yourself, public-domain classics, files from DRM-free retailers — convert without any restriction.
Most files finish in well under a minute. Small images and documents are typically ready in a few seconds; large video or audio files scale roughly with duration. Upload speed from your network is usually the dominant factor, not server time.
Yes. The cover is extracted from the MOBI and re-embedded in the FB2 at device-appropriate dimensions. You can also override it in Advanced by uploading a custom cover image alongside the book file.
Mobipocket eBook
Source formatMOBI is the Mobipocket eBook format historically used by Amazon Kindle devices. While Amazon has moved to newer formats, MOBI remains relevant for older Kindles and legacy eBook libraries.
FictionBook
Target formatFictionBook (FB2) is an XML-based eBook format popular in Russia and Eastern Europe. It provides structured semantic markup for fiction and non-fiction books.
Going the other way? Convert FB2 back to MOBI with the same engine.
Common Use Cases
Share across platforms
Send FB2 files to anyone without worrying about whether they have the right software for MOBI.
Embed in documents
Drop FB2 output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
FB2 often produces smaller files than MOBI for web, email and storage.
Archive & future-proof
Store in a widely-supported format that will still open on future operating systems without legacy plugins.
MOBI vs FB2 — Typical file sizes
Approximate file sizes for common scenarios.
- Text-only novel 200 KB - 1 MB
- Illustrated book 2-10 MB
- Novel (text only) 200-800 KB
- Novel with cover image 300 KB - 1.5 MB
- Illustrated children's book 5-30 MB
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Related comparisons
See these formats side by side to understand which fits your use case best.
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