.htmlz

File format · Released 2010 · By Calibre (Kovid Goyal)

About HTMLZ Files

HTMLZ eBook

HTMLZ 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.

Family

eBooks

Extension

.htmlz

MIME Type

application/htmlz

Can Use As

Input Output

● Hex Magic-Bytes Signature (Offset 0x00) eBooks Binary/Text Stream

48 54 4D 4C 00 01 00 00 4B 41 49 4A ASCII: HTML....KAIJ

Convert HTMLZ → PDF → 15+ CONVERSIONS ↓

WASM + ISO-27001 HYBRID ENGINE ⚡ 0ms Local Browser Mode

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📏 Physical Size (CM / MM / Inch → 300 DPI Pixels)

Width ()

Height ()

Unit

Print Density (DPI)

Passport & ID Presets:

📈 Target Minimum File Size (Guaranteed ≥ KB)

Lossless JPEG Q100 + standard COM segment padding for government & exam portals

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The story

HOW HTMLZ CAME TO BE.

HTMLZ — zipped HTML — is a minor ebook format defined by Calibre, the open-source ebook management tool by Kovid Goyal. An .htmlz is literally a ZIP archive containing HTML files plus CSS and image assets. Think of it as "EPUB lite" — the same ZIP-of-HTML structure but without EPUB’s required container.xml or spine manifest.

HTMLZ is useful when you want an ebook-like package without ebook-reader-specific metadata. Calibre can import almost anything to HTMLZ for lightweight intermediate work. Most users never consciously use HTMLZ — but it occasionally surfaces in ebook conversion workflows.

ISO / MIME SPECS

Technical Specifications

MIME type: application/x-htmlz
Extension: .htmlz
Container: ZIP + HTML
Tool: Calibre

Typical Sizes & Weights

Typical novel

300 KB - 2 MB

STRENGTHS & LIMITATIONS.

Strengths

Simpler than EPUB.
ZIP-of-HTML portability.
Calibre-native.

Limitations

Niche — no reader support.
Not a mainstream delivery format.
Calibre-only.

CURIOSITIES & TRIVIA.

HTMLZ is literally a ZIP of HTML files — rename to .zip and you can browse the book's chapters directly.

Calibre defined HTMLZ as a lightweight alternative to EPUB for intermediate conversion workflows.

Kovid Goyal created Calibre in 2006 as a one-man project — it grew to support 25+ ebook formats.

HTMLZ lacks EPUB's mandatory container.xml — simpler but non-standard.

Almost no dedicated ebook reader supports HTMLZ natively — it's a Calibre ecosystem format.

STUDIO TOOLS FOR HTMLZ

Specialized Utilities for .HTMLZ Files

View all 45 tools →

merge →

Merge PDF

Combine multiple PDF files into a single document. Drag and drop to reorder pages before merging. No software installation required.

split →

Split PDF

Split a PDF into separate files by page ranges or extract individual pages. Perfect for isolating chapters, invoices, or specific sections.

compress →

Compress PDF

Reduce PDF file size without visible quality loss. Choose from low, medium, or high compression levels. Ideal for email attachments and web uploads.

convert →

PDF to Word Converter

Convert PDF documents into editable Microsoft Word (DOCX) files online for free while preserving layout, tables, and typography.

convert →

Word to PDF Converter

Convert Microsoft Word documents (DOCX, DOC) into print-ready PDF files with exact font embedding and layout preservation.

convert →

PDF to Excel Converter

Extract tables, financial rows, and structured columns from PDF files directly into editable Microsoft Excel (XLSX) spreadsheets.

ocr →

Make PDF Searchable (OCR)

Add a searchable text layer to scanned PDF documents using OCR. The visual appearance stays the same but you can now search, select, and copy text.

decrypt →

Unlock PDF

Remove owner printing/copying restrictions and decrypt password-protected PDF files instantaneously once you enter the known password.

CONVERT FROM HTMLZ

htmlz pdf htmlz docx htmlz odt htmlz txt htmlz md htmlz html htmlz epub htmlz mobi htmlz azw3 htmlz fb2 htmlz rst htmlz txtz htmlz pdb htmlz lrf htmlz snb

CONVERT TO HTMLZ

epub htmlz mobi htmlz azw3 htmlz fb2 htmlz txtz htmlz pdb htmlz lrf htmlz snb htmlz lit htmlz cbz htmlz cbr htmlz

Common Use Cases

Calibre intermediate format, ebook conversion pipelines, HTML-based ebooks