.chm
CHM files: what they are and how to open them
A CHM file is a compiled Windows help book: many HTML pages in one file, with a contents tree and search. Read one offline on Android or Windows in Universal File Opener, without the Windows HTML Help viewer.
Check your CHM file here
Nothing is uploaded. This page names the type and shows its details; to view it, use an app below.
What is a CHM file?
CHM stands for Compiled HTML Help. Microsoft introduced it in the late 1990s for Windows program help. Many older manuals, API references and ebooks come as CHM.
Windows opens a CHM with its built-in HTML Help viewer, but often shows blank pages for a file downloaded from the internet or kept on a network drive, because Windows blocks it. Mac and Android have no CHM reader of their own.
What is inside
A CHM starts with ITSF. Inside, a directory lists the book’s files and where each sits, and the content is compressed with LZX. The HTML pages, pictures and style sheets are ordinary web files. A .hhc file holds the contents tree and a .hhk file the keyword index.
Universal File Opener reads the directory, unpacks pages as you open them and shows the contents, pictures and search. If the contents file is missing it lists pages in directory order. Scripts, network loads and external links are blocked.
manual.chmITSF header├─ directoryevery file and its position├─ index.html …pages, LZX compressed├─ images, stylespictures and CSS├─ contents.hhcthe contents tree└─ index.hhkthe keyword indexHow to open a CHM file
Windows
- HTML Help viewerBuilt into Windows. Opens a CHM once it is unblocked: right-click the file, choose Properties, then Unblock.
- 7-ZipFree. Opens a CHM as an archive so you can pull out its HTML pages.
- Universal File Opener for WindowsOursReads the book offline with contents, search and pictures, even when the file came from the internet. Read only.
Mac
- CalibreFree. Reads and converts CHM books to EPUB or PDF.
Android
- Universal File OpenerOursReads the book offline with contents, search and pictures. Free to open and read only.
In a browser
- Edge or ChromeBrowsers cannot open a .chm file. Extract its pages with 7-Zip first, then open the start page, often index.html.
Won’t open?
Pages are blank or say the page was canceled
Windows blocks CHM files from downloads and network drives. For a download, right-click the file, choose Properties and Unblock. Copy a file on a network drive to a local folder first. Or open it in Universal File Opener.
Links to websites do nothing
Universal File Opener blocks scripts, network loads and external links inside a CHM. Copy the address into a browser.
The contents tree is missing
If the book’s contents file cannot be read, the app lists the pages in directory order instead.
Only the first part of a huge book appears
Books with more than 5,000 chapters show a bounded first part with a notice. Extract the pages with 7-Zip for the rest.
The app refuses the file
Malformed or unsupported structures are refused, as are books beyond 64 MiB. Try 7-Zip to extract what survives.
Convert a CHM file
- To EPUB or PDFIn Calibre, add the book, choose Convert books and pick EPUB or PDF. Check the result for broken links.
- To HTML pagesIn 7-Zip choose Extract here. In Universal File Opener use Extract to save the book’s files.
CHM files in our apps
Universal File Opener reads CHM help books offline on Android and Windows, with chapters, the contents tree, search and pictures. Scripts, network loads and external links are blocked. Books are read only.
All Android tools and limits for CHM files
- Reads chapters, the contents tree, pictures and text search, and can extract the book's files.
Limits
- Books up to 64 MiB open; more than 5,000 chapters show a bounded first part with a notice.
- Scripts, network loads and external links are blocked. Malformed or unsupported structures are refused.
- No editing and no conversion.