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ARJ file format

.arj  ·  archive+compression  ·  introduced 1991 by Robert K. Jung

What is a ARJ file?

ARJ (Archived by Robert Jung) was an extremely popular DOS archiver in the early 1990s, prized for slightly better compression than ZIP and built-in support for splitting archives across floppy disks. It has been almost entirely replaced by ZIP, RAR, and 7z but is still encountered when handling old shareware and BBS downloads.

Most readers reach this page after double-clicking a .arj file and finding their operating system unsure what to do with it. The good news: ARJ 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 ARJ file

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

On Windows

  1. Locate the .arj file in File Explorer.
  2. 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…
  3. If prompted, pick a destination folder and click Extract.
  4. The extracted files appear in a folder next to the original archive.

On macOS

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

On Linux

  1. Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).
  2. On the command line, run one of: 
    7z x archive.arj    # works for almost any format
  3. The files appear in the current directory.

How to create a ARJ file

To pack files into a .arj 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 ARJ.

On the command line, the canonical create command is:

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

  • Multi-volume floppy support
  • Better ratio than early ZIP

Weaknesses and limitations

  • Proprietary
  • Effectively obsolete
  • Encryption is weak

Typical use cases

  • Legacy DOS archives
  • Vintage shareware distribution

Technical details

ARJ uses the ARJ proprietary (LZ77 family) algorithm and is identified on disk by the magic byte sequence 60 EA. The standard MIME type is application/x-arj. The format is a proprietary specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is ARJ safe to open?

Archive files are containers — they are only as safe as what they contain. A .arj 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 ARJ?

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

How does ARJ 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.

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