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ISZ file format

.isz  ·  disk image  ·  introduced 2007 by EZB Systems (UltraISO)

What is a ISZ file?

ISZ is a compressed and optionally encrypted ISO container introduced by UltraISO. It is essentially an ISO that has been chunked, compressed with zlib or bzip2, and wrapped with an index for random access. Mounting an ISZ in UltraISO behaves identically to mounting an ISO but uses far less disk space.

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

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

On Windows

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

How to create a ISZ file

To pack files into a .isz 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 ISZ.

On the command line, the canonical create command is:

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

  • Random-access decompression
  • Optional AES-256
  • Smaller than raw ISO

Weaknesses and limitations

  • Proprietary
  • UltraISO/PowerISO ecosystem

Typical use cases

  • Compressed ISO archives
  • Encrypted disc backups

Technical details

ISZ uses the zlib / bzip2 / AES-256 algorithm and is identified on disk by the magic byte sequence 49 73 5A 21. The standard MIME type is application/x-isz. The format is a proprietary specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is ISZ safe to open?

Archive files are containers — they are only as safe as what they contain. A .isz 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 ISZ?

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

How does ISZ 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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