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.umod / .upk (Unreal mod / package) file format

.umod · .upk · .pak  ·  archive (game asset bundle)  ·  introduced 1998 by Epic Games

What is a .umod / .upk (Unreal mod / package) file?

Unreal Engine games ship their assets in .pak (modern UE4/UE5) or .upk (UE3) archives. UE5 typically uses Oodle compression — extremely fast decompression that lets the engine stream texture and mesh data on demand. Modders use UE Viewer (umodel) to extract assets for analysis and conversion.

Most readers reach this page after double-clicking a .umod file and finding their operating system unsure what to do with it. The good news: .umod / .upk (Unreal mod / package) 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 .umod / .upk (Unreal mod / package) file

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

On Windows

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

How to create a .umod / .upk (Unreal mod / package) file

To pack files into a .umod 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 .umod / .upk (Unreal mod / package).

On the command line, the canonical create command is:

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

  • Designed for streaming
  • Multiple compressors over time
  • Huge ecosystem

Weaknesses and limitations

  • Format revisions across UE versions
  • Some games encrypt their .pak files

Typical use cases

  • Unreal Engine games
  • Game modding and asset extraction

Technical details

.umod / .upk (Unreal mod / package) uses the zlib / LZX / LZ4 / Oodle algorithm and is identified on disk by the magic byte sequence C1 83 2A 9E. The standard MIME type is application/x-unreal. The format is an open specification and is free for any use.

Frequently asked questions

Is .umod / .upk (Unreal mod / package) safe to open?

Archive files are containers — they are only as safe as what they contain. A .umod 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 .umod / .upk (Unreal mod / package)?

Some archivers can wrap .umod / .upk (Unreal mod / package) files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does .umod / .upk (Unreal mod / package) 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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