What is CHM?

CHM (Compiled HTML Help) files are Microsoft's proprietary format for software help documentation, combining multiple HTML pages, images, CSS, and JavaScript into a single compressed binary file. CHM includes a table of contents, index, full-text search, and navigation tree. Based on Internet Explorer rendering engine, CHM files display HTML content in a standardized help viewer window. Created with Microsoft HTML Help Workshop.

CHM is widely used for Windows application help files, software documentation, technical manuals, and eBook distribution (especially technical books). Common in legacy Windows applications and still encountered in many older software packages. While Microsoft has moved to newer formats, CHM remains prevalent due to millions of existing help files. Works natively on Windows; requires third-party tools (xCHM) on Linux/Mac.

Did you know? CHM files use LZX compression - the same as CAB archives!

History

Microsoft developed CHM to replace the aging WinHelp (.hlp) format with a modern HTML-based help system integrated with Internet Explorer.

Key Milestones

  • 1997: HTML Help 1.0 released
  • 1998: Windows 98 integration
  • 2000: Widespread adoption
  • 2006: Vista transitions away
  • 2012: Microsoft deprecates format
  • Present: Legacy support only

Key Features

Core Capabilities

  • Compiled Format: Single file package
  • Full-Text Search: Indexed searching
  • Table of Contents: Hierarchical navigation
  • Index: Keyword index
  • Compression: LZX compression
  • HTML Rendering: Rich formatting

Common Use Cases

Software Help

Application documentation

Technical Docs

API documentation

Training

User manuals

eBooks

Technical book distribution

Advantages

  • Single file distribution
  • Excellent full-text search
  • Built-in table of contents
  • Good compression
  • Native Windows support
  • Rich HTML formatting
  • Widely supported legacy format

Disadvantages

  • Deprecated by Microsoft
  • Windows-only (natively)
  • Security vulnerabilities
  • IE rendering engine dependency
  • Not suitable for web
  • Limited cross-platform support

Technical Information

Format Specifications

Specification Details
File Extension .chm
MIME Type application/vnd.ms-htmlhelp
Compression LZX algorithm
Contents HTML, CSS, JS, images
Rendering Internet Explorer engine
Status Deprecated by Microsoft

Common Tools

  • Windows: Built-in HH.exe viewer
  • Creation: Microsoft HTML Help Workshop
  • Cross-platform: xCHM (Linux/Mac), CHM Viewer
  • Conversion: CHM to PDF converters, CHM Decoder