﻿ CHM File Extension - What is it and how to open CHM format

Opening CHM files

Have a problem opening a .CHM file? We collect information about file formats and can explain what CHM files are. Additionally we recommend software suitable for opening or converting such files.

What is the .CHM file type?

The .chm filename extension is associated with the Compressed, or Compiled HTML (CHM) file format and type. CHM is a Windows-specific format, developed by Microsoft and previously intended as a single standard format for Microsoft Windows help files. CHM retains its legacy status, having given way to the newer MAML format.

A .chm file is a help or e-book file in the CHM format. The file contains a set of HTML pages, hierarchically interconnected with an index and cross-referencing hyperlinks. CHM files use the LZX compression algorithm. All valid .chm files have the "ITSF" signature in their header.


CHM files downloaded from the Internet or supplied with third-party applications will often fail to open properly on Microsoft Windows (Vista and later), as they are blocked by the Windows security system by default. To open such a blocked .chm file, one has to "unblock" it in the file's properties ("Properties"–"General"–"Unblock").

Beside the native support in Windows (since Windows 98/NT), there are also multiple Windows and non-Windows tools that can open CHM files and display, edit, or convert their contents.

Software to open or convert CHM files

You can open CHM files with the following programs: ﻿

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