ARJ (ARJ Archive)
A DOS-era compression format that packed entire software libraries onto floppy disks before ZIP became the default.
| Full name | ARJ Archive |
| Extension | .arj |
| MIME type | application/x-arj |
| Developer | Robert K. Jung |
| Released | 1991 |
| Type | Archive / lossless compression |
| Compression | Proprietary LZ-based (AR001 / AR002) |
| Encryption | Basic password protection (weak by modern standards) |
What is a ARJ file?
ARJ is a compressed archive format created by Robert K. Jung and first released in 1991. The name stands for Archived by Robert Jung. It was widely used on DOS and early Windows systems to distribute software and share files during the 1990s.
A .arj file is a single compressed package that holds one or more files and folders together. It uses a proprietary LZ-based compression algorithm called AR001 or AR002, which typically achieves compression ratios similar to ZIP. The format supports multi-volume archives, meaning a large archive can be split across several smaller files, which was essential in the floppy-disk era. It also includes basic password protection, though the encryption is not strong by today's standards.
History
Robert K. Jung developed ARJ in 1990 and released it publicly in 1991 as shareware for MS-DOS. During the early 1990s it competed directly with PKZIP and became popular for software distribution and BBS (bulletin board system) file sharing. ZIP eventually dominated the market due to broader tooling support, but ARJ remained in active use through the late 1990s; Jung later released ARJ 3.x for Windows with 32-bit and 64-bit support.
How it works
An ARJ file begins with a main archive header that contains the archive name, timestamps, and format version fields. Each compressed file inside the archive is preceded by a local file header storing the original filename, size, CRC-32 checksum, and compression method. The actual compressed data immediately follows each local header. Multi-volume archives use continuation markers in the main header so extraction tools know which volume comes next.
What it is used for
- Extracting old software distributed on floppy disks or downloaded from 1990s BBSes
- Accessing archived files from legacy DOS and early Windows environments
- Converting legacy ARJ archives into modern formats like ZIP or 7Z for easier sharing
- Preserving and migrating historical software collections in archival projects
How to open it
7-Zip (free, Windows and Linux) can open .arj files natively using its built-in ARJ support. On macOS, The Unarchiver handles .arj files and is available as a free download from the App Store.
Pros and cons
Strengths
- Multi-volume archive support made it practical for floppy-disk software distribution
- Compression ratios comparable to ZIP on typical files from the 1990s
- CRC-32 checksums on every file detect corruption during extraction
- Still readable by modern tools like 7-Zip, so old archives remain accessible
Trade-offs
- Password protection uses weak encryption that does not meet modern security requirements
- Largely obsolete: ZIP, 7Z, and RAR offer better compression and broader native support
- No native support in Windows Explorer or macOS Finder; requires a third-party tool
- Solid compression is not available, so compression ratios lag behind modern formats on many-small-files archives
Convert ARJ files
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ARJ FAQ
What does ARJ stand for?
ARJ stands for Archived by Robert Jung, after its creator Robert K. Jung who released the format in 1991.
Can I open an ARJ file on Windows 11 or macOS without extra software?
No. Neither Windows nor macOS includes built-in ARJ support. You need a free tool like 7-Zip on Windows or The Unarchiver on macOS to extract the contents.
Is ARJ still used today?
Rarely. ARJ is mainly encountered when accessing old software archives from the 1990s, such as files downloaded from early BBSes or bundled on vintage floppy-disk software collections. For new archives, ZIP, 7Z, or RAR are better choices.
Is it safe to trust the password protection in an ARJ archive?
No. ARJ uses a simple proprietary encryption scheme that is considered weak by modern standards and can be broken with basic tools. Do not rely on ARJ password protection to secure sensitive data.