What is a CHM file?
CHM stands for Compiled HTML Help. It was created by Microsoft and first released in 1997. CHM packs a whole website of help pages into one compressed file with a contents tree and search index. Windows still opens it, but blocks CHM files downloaded from the internet until you tick Unblock in their Properties, because they can run scripts.
| Type | e-book |
|---|---|
| Compression | Lossless |
| In a browser | opens with a double-click on Windows; a Mac or Linux PC needs an app such as Calibre or xCHM |
| Common uses | Windows help files, software manuals and some older e-books |
| MIME type | application/vnd.ms-htmlhelp |
How to open a CHM file
A CHM file does not open in a web browser, and only some devices have a program for it. Here is what works on each. If none of these is to hand, converting the file to PDF here makes it open on any device.
| Windows | Double-click it: Windows opens CHM files in its built-in help viewer. For a file from the internet, tick Unblock in its Properties first. |
|---|---|
| Mac | Calibre and iChm open it. |
| iPhone and iPad | iPhone and iPad have no CHM viewer. Convert it to PDF or EPUB here to read it. |
| Android | A few reading apps open CHM; converting it to PDF or EPUB here is simpler. |
Want the technical details? See what a .chm file is, with its MIME type, file signature and a viewer.
CHM compared with other e-book formats
How CHM stacks up against the formats people most often convert it to or from:
| Format | Compression | Usually contains | Released | Best for |
|---|---|---|---|---|
| CHM | Lossless | HTML pages, images, a contents tree and a search index in one compressed file | 1997 | Windows help files, software manuals and some older e-books |
| EPUB | Lossless | chapters as XHTML pages, with images and a table of contents | 2007 | e-books from stores other than Amazon and free e-book libraries |
| AZW3 | Lossless | a book's text, images, styling and table of contents | 2011 | Kindle e-readers and books loaded onto them over USB |
| MOBI | Lossless | a book's text, images and table of contents | 2000 | older Kindle e-readers and e-books from the 2000s |
| FB2 | Lossless | a book's text, structure and metadata as a single XML file | 2004 | e-books from Russian and Eastern European libraries |
How to use the CHM converter
- Choose the format you want in the Convert to menu.
- Drop your CHM files on the box, or press Choose CHM files. Several files can go in at once.
- Adjust the settings if you need to; the defaults suit most people.
- Press Convert and save each file with its Download button. The file is converted on our server and deleted as soon as the result is sent back.
Tips for converting CHM files
Converting a CHM file keeps the book's text, chapters and images; what changes is how it is packaged for each reader. A few habits give the best result.
- For a current Kindle, send EPUB through Send to Kindle, or copy AZW3 over USB. MOBI is for older Kindles.
- Books with DRM cannot be converted. DRM-free books from Project Gutenberg, Standard Ebooks and independent authors convert normally.
- Keep the original file; each conversion rebuilds the book.
- For PDF to EPUB, a PDF made from a text document converts far better than a scan or a multi-column layout.
- Calibre, the engine used here, is free for your own computer too, and handles whole libraries at once.
Convert CHM files from the command line
7-Zip unpacks a CHM into the web pages inside it, which any browser opens and can print to PDF:
7z x input.chm -ohelpConverting on your own computer keeps the file entirely local, which is worth it for very large or very sensitive documents. For everything else, the converter on this page does it without installing anything.
CHM converter FAQ
What is a CHM file?
CHM stands for Compiled HTML Help, an e-book format created by Microsoft in 1997. CHM packs a whole website of help pages into one compressed file with a contents tree and search index. Windows still opens it, but blocks CHM files downloaded from the internet until you tick Unblock in their Properties, because they can run scripts.
How do I open a CHM file?
CHM opens with a double-click on Windows; a Mac or Linux PC needs an app such as Calibre or xCHM. If the program you need will not open it, convert it to a more widely supported format above.
What is the most common CHM conversion?
CHM to EPUB is the conversion people search for most. The CHM is unpacked by 7-Zip on our conversion server, and its pages are joined into one document in the order of the help file's own table of contents.
Does converting CHM files here upload them?
Yes. CHM conversions need 7-Zip, pandoc and Chromium, open-source programs that cannot run in a browser, so they run on our conversion server. The file is uploaded, converted, and deleted together with the result as soon as the result has been sent back to you. Nothing is kept.
Is the CHM converter free?
Yes. There is no sign-up and no watermark. Conversions that use our server accept files up to 25 MB, with a fair-use limit per hour. The site is supported by advertising.
Can I convert CHM files on iPhone or Android?
Yes. Open this page in Safari on iPhone and iPad, or in Chrome on Android, choose your CHM files and download the results to your Downloads folder.
Can I convert several CHM files at once?
Yes. Drop as many as you like; they are converted one after another, each with its own download button.