CONVERT
MOBI → HTMLZ
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Fast, secure MOBI to HTMLZ conversion. No registration required.
Situation. MOBI is Amazon's legacy Mobipocket ebook format, the older Kindle delivery format. Solution: a HTMLZ, produced below. A HTMLZ converted from a MOBI is a new ebook container with the same content inside. The text body survives losslessly; formatting survives as closely as the HTMLZ container supports it; cover images are re-embedded at the target's recommended dimensions. A quick refresher — MOBI is Amazon's legacy Mobipocket ebook format, the older Kindle delivery format. By contrast, HTMLZ is an ebook-family format used for digital reading on e-readers and reading apps.
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.
HTMLZ eBook
Target formatHTMLZ is a zipped HTML ebook format used by Calibre as a lossless intermediate representation. It packages HTML content, CSS stylesheets, and images into a single ZIP archive, preserving full formatting fidelity during ebook conversion chains.
Why convert MOBI to HTMLZ
Library applications (Calibre, Apple Books, Kindle library) prefer a single format for indexing. Converting mixed MOBI/HTMLZ collections to one canonical HTMLZ makes search, filter and sync work reliably.
HOW TO CONVERT
MOBI → HTMLZ
Start the job
Upload a MOBI; we read metadata, cover image and table of contents from the source container.
Rewrite as HTMLZ
The content is flowed into the HTMLZ format with appropriate CSS, images re-encoded at recommended dimensions.
Save the ebook
Download the HTMLZ. Side-load to your device or drop into your reader library.
Common Use Cases
Share across platforms
Send HTMLZ files to anyone without worrying about whether they have the right software for MOBI.
Embed in documents
Drop HTMLZ output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
HTMLZ 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 HTMLZ — 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.
HTMLZ Strengths
- Simpler than EPUB.
- ZIP-of-HTML portability.
- Calibre-native.
Limitations
- Niche — no reader support.
- Not a mainstream delivery format.
- Calibre-only.
MOBI vs HTMLZ — 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)
HTMLZ
- MIME type
- application/x-htmlz
- Extension
- .htmlz
- Container
- ZIP + HTML
- Tool
- Calibre
| Specification | MOBI | HTMLZ |
|---|---|---|
| MIME type | application/x-mobipocket-ebook | application/x-htmlz |
| Extension | .mobi, .prc (PalmDOC variant) | .htmlz |
| Container | Palm Database (PDB) | ZIP + HTML |
| Markup | Compressed HTML subset | — |
| Successor | .azw, .azw3 (Kindle-specific) | — |
| Tool | — | Calibre |
MOBI vs HTMLZ — Typical file sizes
Approximate file sizes for common scenarios.
MOBI
- Text-only novel 200 KB - 1 MB
- Illustrated book 2-10 MB
HTMLZ
- Typical novel 300 KB - 2 MB
Quality & Compatibility
DRM-protected MOBI files cannot be converted — KaijuConverter respects publisher encryption and returns an error rather than stripping protection. Unprotected or self-published MOBI ebooks convert without restriction.
Tips for Best Results
- Use structure detection (TOC > chapters) to regenerate a clean table of contents in the HTMLZ if the MOBI 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 HTMLZ cover rather than auto-generating per MOBI.
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 HTMLZ. 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 HTMLZ 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 HTMLZ at device-appropriate dimensions. You can also override it in Advanced by uploading a custom cover image alongside the book file.
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Read guideSecure & Private Conversion
Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 60 minutes. We never read, share, or store your data.