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.CHM file extension

To open .CHM files on Windows, double-click the .CHM file to open it in the Windows HTML Help viewer (if associated).

To open a .CHM file, use a CHM/HTML Help viewer—on Windows, Microsoft’s built-in HTML Help viewer typically opens it by default. If it doesn’t open, choose a trusted CHM-capable viewer or transfer the file to a desktop OS for the best experience.

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Last updated: April 29, 2026 · Reviewed by Julian Stricker

Open on your device

Choose your operating system for a dedicated step-by-step opening guide.

How to open .CHM files

Use these platform-specific instructions to open .CHM files safely.

Windows

  1. Double-click the .CHM file to open it in the Windows HTML Help viewer (if associated).
  2. If it doesn’t open, right-click the file → Open with → choose a CHM/HTML Help viewer available on your system.
  3. If the file is blocked or shows missing content, copy it to a local folder (e.g., Documents) and try again.
Full Windows guide

Mac

  1. Try opening the .CHM file with a CHM viewer app for macOS (if you have one installed) via Control-click → Open With.
  2. If you don’t have a CHM viewer, transfer the file to Windows or Linux where CHM support is more common.
Full Mac guide

Linux

  1. Open the file from your file manager; if no app is suggested, use Open With and pick a CHM-capable viewer if installed.
  2. If the desktop doesn’t recognize it, check that the file is identified as application/vnd.ms-htmlhelp and install/associate a CHM viewer for your environment.
Full Linux guide

iOS

  1. CHM viewing is not consistently supported natively; if the Files app can’t preview it, share it to a CHM-capable reader app (if available) or open it on a desktop OS.
Full iOS guide

Android

  1. If your file manager can’t open it, try opening it with an installed CHM reader app; otherwise transfer it to Windows/Linux for reliable viewing.
Full Android guide

Security notes

  • CHM files can contain active HTML content; treat CHM files from untrusted sources as potentially risky and avoid opening unknown CHM attachments.
  • Be cautious when a CHM file contains links to external resources or prompts you to run or download other files—CHM is a help container, and unexpected behavior is a red flag.
  • On Windows, security restrictions may block some CHM content when the file comes from the internet or a network location; only unblock and open if you trust the origin.

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Can't open this file?

These are the most common causes and fixes when .CHM files fail to open.

Common reasons

  • The .CHM file won’t open (or opens in the wrong app)
  • Blank pages, missing navigation/search, or links don’t work
  • Windows shows a security warning or content is blocked
  • File is corrupted or incomplete

Fix steps

  1. On Windows/Linux, use “Open with” to select a CHM-capable viewer and set it as the default if desired.
  2. On macOS/mobile, use a CHM viewer app if available; otherwise open the file on Windows where support is most common.

OS-specific troubleshooting

What is a .CHM file?

.CHM (Microsoft Compiled HTML Help) is a proprietary help format that compiles multiple HTML pages and related assets into a single compressed/binary file with an index and navigation. It is produced by Microsoft’s HTML Help compiler (part of HTML Help Workshop) from a help project and HTML content. On many systems it is associated with the MIME type application/vnd.ms-htmlhelp.

Background

CHM is best known as Microsoft’s compiled help format used to ship product documentation and offline help. A CHM file typically contains a collection of HTML pages along with a table of contents, an index, and search metadata, packaged into one file for easy distribution.

On Windows, CHM files are commonly opened with the system’s HTML Help viewer (hh.exe). Authors historically generated CHM files using Microsoft HTML Help Workshop, which compiles a help project and its HTML assets into a .chm output.

On Linux desktops, CHM files are often recognized via shared MIME info mappings to application/vnd.ms-htmlhelp, but whether it opens when you click it depends on installed viewer applications and your desktop environment’s file associations. On macOS and mobile platforms, CHM viewing is less consistent; you may need a third-party viewer app or to open the content on a desktop OS.

Common MIME types: chemical/x-chemdraw

Further reading

Authoritative resources for more details on the .CHM format.

Common .CHM issues

The .CHM file won’t open (or opens in the wrong app)

File association may be missing or your device may not have a CHM/HTML Help viewer installed.

  1. On Windows/Linux, use “Open with” to select a CHM-capable viewer and set it as the default if desired.
  2. On macOS/mobile, use a CHM viewer app if available; otherwise open the file on Windows where support is most common.

Blank pages, missing navigation/search, or links don’t work

Some viewers have partial CHM feature support, so the TOC, index, or certain internal links/scripts may not behave the same as in the Windows HTML Help viewer.

  1. Try opening the same .CHM file in the Windows HTML Help viewer to confirm whether the file itself is OK.
  2. If you must use another platform, try a different CHM viewer there (feature support varies by application).

Windows shows a security warning or content is blocked

Windows can restrict CHM content (especially when downloaded) which may prevent scripts or some internal content from loading correctly.

  1. Move the .CHM to a local folder (not a network share) and try again.
  2. If you trust the source, use Windows file properties (where available) to unblock, then reopen.

File is corrupted or incomplete

Because CHM is a compiled container, a partial download or truncation can prevent it from opening or cause missing sections.

  1. Re-download or re-copy the file from the original source.
  2. Test the file on another machine/viewer to rule out a device-specific viewer problem.

FAQ

What does .CHM stand for?

It stands for Microsoft Compiled HTML Help, a format that bundles many HTML help pages (plus navigation/search data) into one file.

What MIME type is used for .CHM files?

The registered media type is application/vnd.ms-htmlhelp.

Can I make a .CHM file myself?

Yes. Microsoft’s HTML Help compiler (part of HTML Help Workshop) compiles a help project and HTML assets into a .chm file.

Can I convert a .CHM by renaming the extension?

No. Renaming only changes the filename, not the file’s internal format. Use a proper tool/workflow that exports or compiles help content.

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