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PAQ8 file format

.paq8  ·  compression (single file)  ·  introduced 2002 by Matt Mahoney

What is a PAQ8 file?

PAQ is a series of experimental compressors written by Matt Mahoney that consistently top the Hutter prize and the Large Text Compression Benchmark. They use context mixing — combining predictions from many statistical models — to produce the smallest files of any general-purpose compressor, but at extreme CPU cost. PAQ is for compression researchers, not daily use.

Most readers reach this page after double-clicking a .paq8 file and finding their operating system unsure what to do with it. The good news: PAQ8 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 PAQ8 file

Opening a .paq8 file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

  1. Locate the .paq8 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 .paq8 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.paq8    # works for almost any format
  3. The files appear in the current directory.

How to create a PAQ8 file

To pack files into a .paq8 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 PAQ8.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .paq8 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

  • Best-in-class ratio
  • Active research community

Weaknesses and limitations

  • Glacially slow
  • High memory
  • Many incompatible variants

Typical use cases

  • Compression benchmarks
  • Archival of small but priceless data

Technical details

PAQ8 uses the Context mixing algorithm and is identified on disk by the magic byte sequence 50 51 38. The standard MIME type is application/x-paq. The format is an open specification and is free for any use.

Frequently asked questions

Is PAQ8 safe to open?

Archive files are containers — they are only as safe as what they contain. A .paq8 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 PAQ8?

Some archivers can wrap PAQ8 files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does PAQ8 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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