What is the SIT file format?
A .sit file is an archive file format created in 1987 by Raymond Lau of Aladdin Systems for the classic Macintosh operating system. Today it is readable on many platforms, although it is no longer popular and seldom created from scratch. .sit archives are sometimes described as _"ZIP files for Mac OS"_, since they served the same role on classic Macintosh that ZIP archives play on Windows. The format is proprietary; ownership passed from Aladdin Systems through Allume Systems to Smith Micro Software, which is not the original creator.
A key strength of .sit was its faithful preservation of Macintosh-specific metadata - HFS type and creator codes, as well as both the data fork and resource fork of every archived file. Generic ZIP could not do this. Archives open with the four-byte ASCII signature SIT! and store data in a big-endian binary layout, with per-file compression (LZ and Huffman variants, and the later "Arsenic" method) and optional password protection. Files were frequently paired with a .hqx BinHex wrapper to survive transport over text-only email and Usenet.
The .sit format was superseded in 2002 by the redesigned .sitx (StuffIt X) format, which uses dynamic compression that adapts to the data supplied and is an entirely different binary layout. Smith Micro discontinued active StuffIt development in December 2019, and modern Mac workflows favour ZIP or .dmg for distribution. Free tools such as The Unarchiver and the unar command-line utility still open legacy .sit archives without difficulty.
Security & safety
RISK: MEDIUMA .sit archive can contain any files, including old Mac executables - extract only from trusted sources and scan the contents. Practical risk today is the SOFTWARE: because StuffIt is discontinued, web searches for a 'StuffIt opener' frequently lead to bundled-adware installers and fake 'free download' portals. Get StuffIt Expander only from stuffit.com or the Mac App Store, and prefer the open-source The Unarchiver/Keka. Classic .sit files copied off a Mac may have lost their resource fork, leaving some old files non-functional even after extraction.
Format details
in a nutshellPrograms that open SIT files
Windows3 apps
StuffIt Expander for Windows
Free Legacy free Expander extracts .sit on Windows. Product is discontinued (no updates), but downloads remain at stuffit.com.
The Unarchiver (unar CLI)
Open-source Use the cross-platform 'unar file.sit' command line to extract classic StuffIt archives on Windows. 
macOS3 apps
StuffIt Expander
Free Drag the .sit onto StuffIt Expander (free on the Mac App Store) to extract. The original native opener. 
The Unarchiver
Free Double-click the .sit; The Unarchiver extracts classic StuffIt (and many other) archives. Best free general option. Technical details
deep spec| Container type | Proprietary binary archive |
| Magic bytes | `SIT!` (hex `53 49 54 21`) at byte offset 0; some variants use `SITD` or `StuffIt` |
| Byte order | Big-endian throughout |
| MIME type(s) | `application/x-stuffit`; also `application/x-sit`, `application/stuffit` |
| Compression | Per-file; LZ/Huffman variants; "Arsenic" (method 15) added in StuffIt 5.x |
| Encryption | Optional per-archive; DES in early versions, RC4 in later versions |
| Integrity | Per-entry CRC checksums for data validation |
| Archive structure | Header + sequential file entries, each holding filename, HFS metadata, optional data fork, optional resource fork, and compression data |
| Fork support | Preserves both HFS data fork and resource fork per archived file |
| HFS metadata | Stores Macintosh type codes and creator codes per entry |
| Associated OS | Classic Mac OS, macOS, Windows (via StuffIt Expander) |
| Successor format | `.sitx` (StuffIt X, 2002) - redesigned binary layout, separate format entirely |
| Developer lineage | Aladdin Systems → Allume Systems → Smith Micro Software; discontinued December 2019 |
| Common transfer wrapper | Often distributed inside `.hqx` (BinHex) encoding for safe transit over text-only email and Usenet |
| Released | 1987 (StuffIt, classic Mac OS) |
| Latest version | Classic SIT format (StuffIt 5.x era); superseded by SITX (StuffIt X) in 2002 |
SIT conversions
Community Q&A
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