Convert CHM to EPUB — Free Online Converter
Convert Compiled HTML Help (.chm) to Electronic Publication (.epub) online for free. Fast, secure document conversion with no watermarks or registrati...
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Upload your .chm file by dragging it into the upload area or clicking to browse.
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About CHM to EPUB Conversion
CHM (Compiled HTML Help) is Microsoft's proprietary help file format that bundles HTML pages, images, and a table of contents into a single compressed archive using LZX compression. CHM was the standard documentation format for Windows applications from Windows 98 through Windows Vista, and millions of CHM files exist as software manuals, programming references, API documentation, and technical guides. EPUB (Electronic Publication) is the international open ebook standard supported by Apple Books, Kobo, Google Play Books, and virtually every reading platform worldwide.
Converting CHM to EPUB transforms legacy Windows help documentation into a modern ebook format that can be read on e-readers, tablets, phones, and any device with an EPUB-compatible reading app. This conversion is essential for preserving and redistributing technical documentation originally created for the Windows Help system.
Why Convert CHM to EPUB?
CHM files have significant cross-platform limitations. macOS blocks CHM files by default as a security measure (they can contain ActiveX controls), Linux requires specialized viewers like xCHM or KchmViewer, and mobile devices have no native CHM support at all. Converting to EPUB makes the documentation accessible on every platform without specialized software or security workarounds.
CHM is also a declining format with no active development. Microsoft deprecated the HTML Help system, and modern Windows applications use web-based help or PDF documentation instead. Converting CHM archives to EPUB preserves the content in a format with guaranteed long-term readability — EPUB is an ISO standard (ISO/IEC 23736) with broad institutional support and active development.
Common Use Cases
- Convert legacy Windows software documentation from CHM to EPUB for reading on tablets and e-readers
- Preserve CHM programming references and API documentation in a cross-platform ebook format
- Make CHM technical guides accessible on macOS and Linux without specialized CHM viewer software
- Convert CHM training materials to EPUB for distribution through corporate learning management systems
- Migrate CHM help file archives to EPUB for long-term preservation in digital libraries
How It Works
Calibre's ebook-convert extracts the CHM archive using its LZX decompression engine, parsing the internal directory structure to identify HTML content pages, images, CSS stylesheets, and the table of contents (HHC file). The HHC file — an XML-based sitemap — is converted to the EPUB navigation document (NCX/nav) with proper heading hierarchy. HTML pages are cleaned and restructured into valid XHTML for the EPUB content documents. Embedded images are extracted and stored in the EPUB package. CSS styling from the CHM is preserved where compatible with EPUB renderers. The output is a valid EPUB 3 package with navigation, content structure, and metadata.
Quality & Performance
The conversion produces high-quality output for text-heavy CHM files. HTML content, headings, paragraphs, lists, code blocks, and tables transfer cleanly to EPUB. Images are preserved at their original resolution. The table of contents hierarchy from the CHM's HHC file maps naturally to EPUB navigation. The main limitations involve CHM-specific features: ActiveX controls, JavaScript-powered search indexes, context-sensitive help hooks, and DHTML behaviors have no EPUB equivalent and are stripped during conversion. For standard technical documentation, the conversion produces a well-structured, navigable ebook.
Device Compatibility
| Device | CHM | EPUB |
|---|---|---|
| Windows PC | Partial | Partial |
| macOS | Partial | Partial |
| iPhone/iPad | Partial | Partial |
| Android | Partial | Partial |
| Linux | Partial | Partial |
| Web Browser | No | No |
Tips for Best Results
- 1Verify the table of contents hierarchy in the EPUB — some CHM files have deeply nested structures that may need simplification
- 2CHM files downloaded from the internet may be blocked by Windows — right-click, Properties, Unblock before converting
- 3For programming references, check that code examples and syntax highlighting transferred correctly
- 4If the CHM contains ActiveX or JavaScript interactivity, those features will not transfer — only static content is preserved
- 5Review image quality in the EPUB — some CHM files contain low-resolution screenshots optimized for 1990s screen resolutions
CHM-to-EPUB conversion rescues legacy Windows help documentation from platform lock-in, producing universally readable ebooks that preserve the content structure and navigation of the original help files.