CONVERT FB2 → MOBI (FB2 to MOBI)
Fast, secure FB2 to MOBI conversion. No registration required.
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Ghostscript + QPDFHOW TO CONVERT FB2 → MOBI
Start the job
Upload a FB2; we read metadata, cover image and table of contents from the source container.
Rewrite as MOBI
The content is flowed into the MOBI format with appropriate CSS, images re-encoded at recommended dimensions.
Save the ebook
Download the MOBI. Side-load to your device or drop into your reader library.
Why this pair exists — FB2 is FictionBook, a popular XML-based open ebook format in Russian-speaking markets. Ergo, the MOBI route. A MOBI converted from a FB2 is a new ebook container with the same content inside. The text body survives losslessly; formatting survives as closely as the MOBI container supports it; cover images are re-embedded at the target's recommended dimensions. One more beat. FB2 is FictionBook, a popular XML-based open ebook format in Russian-speaking markets. Receiving format: MOBI is Amazon's legacy Mobipocket ebook format, the older Kindle delivery format.
Why convert FB2 to MOBI
Library applications (Calibre, Apple Books, Kindle library) prefer a single format for indexing. Converting mixed FB2/MOBI collections to one canonical MOBI makes search, filter and sync work reliably.
FB2 vs MOBI — Technical specifications
Side-by-side comparison of the technical details.
FB2
- MIME type
- application/x-fictionbook+xml
- Extensions
- .fb2, .fb2.zip
- Container
- Single XML file (optionally zipped)
- Standard
- FictionBook community spec (maintained on GitHub)
- Encoding
- UTF-8 (required)
- MIME Type
- application/x-fictionbook+xml
- Extension
- .fb2
- Category
- eBooks
- Full Name
- FictionBook
- Conversion Engine
- CALIBRE
MOBI
- MIME type
- application/x-mobipocket-ebook
- Container
- Palm Database (PDB)
- MIME Type
- application/x-mobipocket-ebook
- Extension
- .mobi, .prc (PalmDOC variant)
- Category
- eBooks
- Full Name
- Mobipocket eBook
- Conversion Engine
- CALIBRE
- Markup
- Compressed HTML subset
- Successor
- .azw, .azw3 (Kindle-specific)
| Specification | FB2 | MOBI |
|---|---|---|
| MIME type | application/x-fictionbook+xml | application/x-mobipocket-ebook |
| Extensions | .fb2, .fb2.zip | — |
| Container | Single XML file (optionally zipped) | Palm Database (PDB) |
| Standard | FictionBook community spec (maintained on GitHub) | — |
| Encoding | UTF-8 (required) | — |
| MIME Type | application/x-fictionbook+xml | application/x-mobipocket-ebook |
| Extension | .fb2 | .mobi, .prc (PalmDOC variant) |
| Category | eBooks | eBooks |
| Full Name | FictionBook | Mobipocket eBook |
| Conversion Engine | CALIBRE | CALIBRE |
| Markup | — | Compressed HTML subset |
| Successor | — | .azw, .azw3 (Kindle-specific) |
FB2 vs MOBI — Strengths and limitations
What each format does best, and where it falls short.
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.
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.
Quality & Compatibility
DRM-protected FB2 files cannot be converted — KaijuConverter respects publisher encryption and returns an error rather than stripping protection. Unprotected or self-published FB2 ebooks convert without restriction.
Tips for Best Results
- Use structure detection (TOC > chapters) to regenerate a clean table of contents in the MOBI if the FB2 has none.
- Embedded fonts bloat file size — consider dropping them in Advanced if your reader uses its own typography anyway.
- For serialised fiction, keep cover art consistent across episodes by re-using one MOBI cover rather than auto-generating per FB2.
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 FB2 itself has a well-formed ToC. Calibre reads the navigation structure and writes an equivalent ToC into the MOBI. 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 FB2 and the MOBI 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 FB2 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 FB2 and re-embedded in the MOBI at device-appropriate dimensions. You can also override it in Advanced by uploading a custom cover image alongside the book file.
FictionBook
Source 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.
Mobipocket eBook
Target 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.
Going the other way? Convert MOBI back to FB2 with the same engine.
Common Use Cases
Share across platforms
Send MOBI files to anyone without worrying about whether they have the right software for FB2.
Embed in documents
Drop MOBI output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
MOBI often produces smaller files than FB2 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.
FB2 vs MOBI — Typical file sizes
Approximate file sizes for common scenarios.
- Novel (text only) 200-800 KB
- Novel with cover image 300 KB - 1.5 MB
- Illustrated children's book 5-30 MB
- Text-only novel 200 KB - 1 MB
- Illustrated book 2-10 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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