---
title: ".chm file — what it is and how to open it"
description: "A .chm file is a Compiled HTML Help. Windows' old help-file format — a compressed bundle of web pages with an index. How to open one on Windows, macOS and Linux, and what to convert it to."
url: https://atoneplace.net/file/chm/
site: "At One Place"
---

# .chm files

**.chm file: Compiled HTML Help**

Windows' old help-file format — a compressed bundle of web pages with an index.

## What opens a .chm file

- **Windows:** Microsoft HTML Help viewer (built in), SumatraPDF, Calibre
- **macOS:** xCHM, Calibre
- **Linux:** xCHM, KchmViewer, Calibre

## What a .chm file is

CHM packs HTML pages, images and a search index into one file, and was the standard help system for Windows software from 1997 onward. Windows opens it with the built-in viewer, but files downloaded from the internet often show up blank until you right-click, choose Properties and tick "Unblock" — a security measure, since CHM can run scripts. On macOS and Linux, xCHM and Calibre read them.

## Converting it

A .chm file is commonly converted to .pdf, .html or .epub. It is a binary format, so a text editor will show unreadable characters rather than its contents.

## Details

- **Extension:** .chm
- **Full name:** Compiled HTML Help
- **Category:** Documents
- **MIME type:** application/vnd.ms-htmlhelp
- **Created by:** Microsoft
- **Format:** Binary

## Common questions

### How do I open a .chm file?

On Windows use Microsoft HTML Help viewer (built in); on macOS use xCHM; on Linux use xCHM. Windows' old help-file format — a compressed bundle of web pages with an index.

### What is a .chm file?

Compiled HTML Help. CHM packs HTML pages, images and a search index into one file, and was the standard help system for Windows software from 1997 onward. Windows opens it with the built-in viewer, but files downloaded from the internet often show up blank until you right-click, choose Properties and tick "Unblock" — a security measure, since CHM can run scripts.

### Can I convert a .chm file?

Yes. It is usually converted to .pdf, .html or .epub, either by exporting from one of the programs listed above or with a format converter.

## Related pages

- [.pdf — Portable Document Format](https://atoneplace.net/file/pdf/)
- [.docx — Word Document (Open XML)](https://atoneplace.net/file/docx/)
- [.xlsx — Excel Workbook (Open XML)](https://atoneplace.net/file/xlsx/)
- [.pptx — PowerPoint Presentation (Open XML)](https://atoneplace.net/file/pptx/)
- [.txt — Plain Text File](https://atoneplace.net/file/txt/)
- [.doc — Word Document (legacy binary)](https://atoneplace.net/file/doc/)
- [.xls — Excel Workbook (legacy binary)](https://atoneplace.net/file/xls/)
- [.ppt — PowerPoint Presentation (legacy binary)](https://atoneplace.net/file/ppt/)
- [.md — Markdown Document](https://atoneplace.net/file/md/)
- [.rtf — Rich Text Format](https://atoneplace.net/file/rtf/)
- [.pages — Apple Pages Document](https://atoneplace.net/file/pages/)
- [.odt — OpenDocument Text](https://atoneplace.net/file/odt/)

## Sources

- Media Types Registry — IANA (https://www.iana.org/assignments/media-types/)
- Official vendor product documentation — At One Place (/about/)