StuffIt file format

.sit · .sitx · archive+compression · introduced 1987 by Raymond Lau / Aladdin Systems

What is a StuffIt file?

StuffIt was the dominant Macintosh archive format from 1987 well into the 2000s. It pioneered specialty compression for JPEG and MP3, recompressing those formats losslessly to smaller sizes. Modern macOS uses ZIP by default and StuffIt is largely a legacy format today.

Most readers reach this page after double-clicking a .sit file and finding their operating system unsure what to do with it. The good news: StuffIt is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a StuffIt file

Opening a .sit file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .sit file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .sit file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.sit    # works for almost any format

The files appear in the current directory.

How to create a StuffIt file

To pack files into a .sit archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write StuffIt.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .sit files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Specialized JPEG/MP3 recompression
Long Mac heritage

Weaknesses and limitations

Proprietary
Largely abandoned
Few modern tools support .sitx

Typical use cases

Vintage Mac software downloads
Pre-2010 Mac shareware

Technical details

StuffIt uses the Proprietary StuffIt + JPEG/MP3 specialized algorithm and is identified on disk by the magic byte sequence 53 49 54 21. The standard MIME type is application/x-stuffit. The format is a proprietary specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is StuffIt safe to open?

Archive files are containers — they are only as safe as what they contain. A .sit from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a StuffIt?

Some archivers can wrap StuffIt files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does StuffIt compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.