What is the CHW file format?
.chw files are binary sidecar index files used by the Windows HTML Help system to support merged CHM help sets.
Files with the .chw extension contain a combined table of contents and full-text search index spanning multiple .chm files. Each individual help document is formatted in HTML and compiled into a .chm file; when a help project merges two or more of these files - via hhc.exe or HTML Help Workshop's MERGE directive - the HTML Help runtime (hhctrl.ocx) automatically generates a .chw alongside the main .chm. The .chw aggregates TOC entries and cross-file topic URL references from all merged files, allowing the help viewer to display a single unified table of contents across the entire documentation set.
A .chw should be regenerated each time a referenced .chm document is modified, or whenever help entries are added or removed from the documentation. If the .chw falls out of sync with its .chm files, reference errors may occur. Deleting the .chw and reopening the main .chm causes the HTML Help runtime to rebuild it automatically - users rarely need to manage this file directly.
Typical .chw files range from a few kilobytes to tens of megabytes and are uncompressed by default. They can be compressed using dedicated tools such as KeyCHW to save storage space if needed.
Security & safety
RISK: LOWA .chw file is a passive binary index with no executable code or scripts. It cannot spread malware or run anything by itself. It is safe to delete - the HTML Help system will regenerate it. The only risk is if you delete it from a multi-CHM help set and the help viewer cannot rebuild it for some reason, in which case the merged table of contents won't work until you reinstall or re-merge the help set.
Format details
in a nutshell- Crestron control system file - Some Crestron home-automation programming tools use .chw for a project or settings file - completely unrelated to Microsoft HTML Help.
Programs that open CHW files
Windows2 apps
HTML Help (hh.exe) - automatic
Built-in The .chw is read automatically by hh.exe when you open the parent .chm. You do not open the .chw directly - open the associated .chm file instead.
FAR (File and Archive Recompressor)
Paid FAR (from HelpWare Group) is a specialised CHM/CHW tool that can inspect and compress the .chw index. Technical details
deep spec| Format type | Binary sidecar index file for merged Microsoft HTML Help sets |
| Byte order | Little-endian |
| Compression | None by default; KeyCHW tool can compress the file |
| Typical file size | A few KB to tens of MB, depending on number of merged CHM files |
| Associated runtime | hhctrl.ocx (HTML Help ActiveX control, Windows) |
| Generation tool | hhc.exe or HTML Help Workshop via MERGE directive; auto-created by the HTML Help runtime |
| Companion files | Always accompanies one or more .chm files in the same help set |
| Internal structure | Binary index tables - aggregated TOC entries and cross-file topic URL references from all merged CHM files |
| Platform | Windows only (HTML Help is a Windows-exclusive subsystem) |
| Auto-regeneration | Deleting the .chw and reopening the main .chm causes hhctrl.ocx to rebuild it automatically |
| Internal format version | HTML Help 1.x sidecar (hhctrl.ocx v4.x-6.x) |
| Magic bytes | None publicly documented; file begins with proprietary binary index structures |
| Superseded by | Microsoft Help Viewer and online docs for modern Windows apps; CHM + CHW remain in use for legacy software |
| Released | 1997 (HTML Help 1.0, with Windows 98; .chw created when multiple .chm files are merged) |
| Latest version | HTML Help 1.4 (ships with current Windows; hhctrl.ocx runtime; no new versions since early 2000s) |
| Specification | www.nongnu.org |
CHW conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about CHW files.
