What is the FAX file format?
Files with the .fax extension are typically used for storing fax documents - scanned, monochrome page images that have been appropriately compressed for transmission and storage. Most .fax files are either TIFF-wrapped Group 3 or Group 4 raster images, or raw CCITT bitstreams produced by fax modems and software such as WinFax. Because the extension is not tied to a single rigid specification, the internal layout varies by producing application.
The format encodes pages as 1-bit (black-and-white) scan lines using CCITT Group 3 (T.4) or Group 4 (T.6) run-length compression. Fax machines and software typically allow users to change document quality before sending. The size of a .fax file is determined by the resolution setting: standard mode uses 204 × 98 dpi, while fine mode doubles the vertical resolution to 196 dpi. Group 3/4 compression is lossless, so resolution - not compression ratio - governs image detail and affects file size.
Converting .fax to TIFF is possible using a number of available tools. XnView MP and IrfanView handle most variants on Windows; ImageMagick works across platforms with a command like convert input.fax output.tif. If the file is TIFF-wrapped internally, simply renaming it to .tif often makes it open in standard image viewers immediately. Fax software such as Windows Fax and Scan can also read and re-export these files directly.
Security & safety
RISK: LOWA .fax is image data, not executable code, so opening one is safe. The only practical issues are non-malicious: some proprietary .fax variants (e.g. WinFax) won't open in standard viewers, and online fax-to-PDF converters upload your document to a third party - a privacy concern for sensitive faxes, so prefer a local tool (XnView/ImageMagick). As always, confirm the file's real extension if it arrived unexpectedly.
Format details
in a nutshell- WinFax PRO fax (FXS/FXR) - Symantec WinFax PRO stored sent/received faxes; some exports/files carry .fax and need WinFax or a G3-aware viewer.
- Raw Group 3 fax bitstream (.g3) - Plain CCITT G3-encoded scan data with no container, opened by ImageMagick/libtiff tools or fax utilities.
Programs that open FAX files
Windows3 apps
IrfanView
Free Open the .fax (install the plugins pack); handles many fax/TIFF variants and batch-converts to PDF. 
XnView MP
Free Open the .fax in XnView - it auto-detects Group 3/4 fax and TIFF-wrapped faxes; export to PDF/TIFF from here. 
Windows Fax and Scan
Built-in Built-in Windows fax tool - can open faxes it manages; for arbitrary .fax files prefer XnView. 
macOS2 apps
Apple Preview
Built-in Double-click - Preview opens TIFF-based .fax files; if it fails, rename to .tif or use XnView. 
Linux2 apps
ImageMagick
Open-source Run 'convert file.fax out.pdf' (or 'display file.fax'); libtiff/G3 support opens most fax images. Technical details
deep spec| Color depth | 1 bpp (black & white bilevel only; no grayscale or color) |
| Compression | CCITT Group 3 (T.4, MH / MR) or Group 4 (T.6, MMR) lossless run-length coding |
| Container | Often TIFF-wrapped; alternatively a raw Group 3 bitstream or vendor-specific container (e.g. WinFax) |
| Standard page width | 1728 pixels per scan line (A4 paper at 204 dpi horizontal) |
| Vertical resolution | 98 lpi normal mode; 196 lpi fine mode |
| MIME type | image/fax (also image/g3fax and image/tiff for TIFF-wrapped variants) |
| Magic bytes | No universal signature; TIFF-wrapped .fax files begin with 49 49 2A 00 (little-endian) or 4D 4D 00 2A (big-endian) |
| Byte order | Little-endian or big-endian when TIFF-wrapped; raw G3 is a bit-oriented stream |
| Multi-page support | Yes - additional pages stored as TIFF Image File Directories when the container is TIFF |
| Error recovery | End-of-line (EOL) marker codes embedded in Group 3 stream; no whole-file checksum |
| Typical file size | 20 KB - 1 MB for a few pages of black-and-white content |
| Encryption | None - data is stored unencrypted |
| Associated platforms | Windows, macOS, Linux (via XnView MP, IrfanView, ImageMagick, GIMP) |
| Superseded by | PDF and TIFF for modern fax document storage and exchange |
| Released | CCITT Group 3 fax standard (T.4) 1980; .fax files from late-1980s/1990s fax software |
| Latest version | ITU-T T.4 / T.6 (Group 3 / Group 4 fax coding) |
| Open standard | Yes · royalty-free |
| Specification | www.itu.int |
FAX conversions
Community Q&A
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