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UZ3 file

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The file extension .uz3 identifies a file, most likely an Unreal package, that was compressed by an Unreal Engine 3 game. It is the equivalent to Unreal Engine 1's UZ file and Unreal Engine 2's UZ2 file. UZ3 files are usually stored on "redirect servers", i.e. HTTP servers the game client can fetch packages from that a game server requires.


The .uz3 format is a general-purpose compression format, but most compression tools limit it to Unreal packages. To compress a file into UZ3, run the command

UT3 Compress path\to\file.ext

from a Windows command prompt. This will create a compressed file in the same directory that has the .uz3 extension appended. (Replace "UT3" with the name of the executable of your game, e.g. "UDK".)

To decompress a UZ3 file, use the command

UT3 Decompress path\to\file.ext.uz3

and the decompressed file with be created in the same directory but with the .uz3 extension removed. (Again, replace "UT3" with your game's executable name.)

Note that the Compress and Decompress commandlets will override any existing file without warning!

There are also 3rd-party tools for compressing or decompressing UZ3 files, like Mc.Gugi's UnrealDeps.

File format[edit]

The format of UZ3 files is relatively simple. It consists of a header and the compressed data:

Field Type Length Value range
UZ3 File Tag int (little-endian) 4 Bytes 0x0000162E (decimal: 5678)
Uncompressed file size int (little-endian) 4 Bytes 0 - 231-1
Compressed data bytes 0 - 231-1 Bytes

The compression format is defined by the Zlib library's compress() and uncompress() functions. The entire file is compressed as a single unit.


  • UnrealDeps by Mc.Gugi - Includes source code for compressing and decompressing uz1, uz2, and uz3 files.