What Is a CHM File?

A CHM file is a Microsoft Compiled HTML Help file. It bundles HTML pages, images, an index, and a table of contents into one compressed help file that opens in the Windows HTML Help viewer.

Quick answer: CHM stands for Compiled HTML Help. Software vendors have used it for years to ship user manuals, offline documentation, and context-sensitive help inside Windows applications.

What is the CHM file format used for?

The CHM format is mainly used for software documentation. A typical CHM help file includes:

Topic pages written in HTML
A table of contents for navigation
An index of keywords
Searchable text for offline lookup
Images, links, and project files such as HHP, HHC, and HHK

That is why people search for terms like what is a CHM file, CHM format, and CHM file format. They are usually trying to open, inspect, or repurpose old Windows help documentation.

How do you open a CHM file?

On many Windows systems, double-clicking a CHM file opens it directly in the built-in Microsoft HTML Help viewer. If the file was downloaded from the internet, Windows may block it for security reasons.

Right-click the CHM file and choose Properties.
If you see an Unblock checkbox or button, enable it.
Open the file again.

If you are on a newer system, read our guide to opening CHM files on Windows 11.

Can you edit a CHM file?

Yes. You can either decompile the file manually and edit the extracted HTML, or use a dedicated editor that handles CHM projects directly. CHM Editor was built for that second workflow.

Viewer only: Good for reading documentation.
Editor: Needed when you want to change text, fix screenshots, update links, or rebuild the help file.
Converter: Useful when you need web-ready HTML, PDF, or Word output.

For hands-on editing instructions, see How to Edit a CHM File.

Can you convert CHM to HTML?

Yes. Because a CHM file is based on compiled HTML content, you can extract or export it to regular HTML pages. The difficult part is preserving navigation, links, images, and topic structure.

We cover the safer workflow in our CHM to HTML conversion guide.

CHM viewer, decoder, decompiler, or editor: what do you need?

These searches often describe different stages of the same job:

CHM viewer: Open and read the file.
CHM decoder or decompiler: Extract the internal HTML files.
CHM editor: Modify content and save changes.
CHM to HTML converter: Publish the documentation in a browser-friendly format.

Pro tip: If you maintain old software documentation, use one tool for viewing, editing, and exporting whenever possible. It reduces broken links, missing images, and rebuild errors.