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UPX (Ultimate Packer for eXecutables) is a free, portable, and high-performance executable packer that supports many popular executable formats. It helps developers and power users reduce file size efficiently while keeping applications easy to distribute and deploy.
This executable packer achieves high compression ratios while maintaining fast decompression performance. Thanks to in-place decompression in most formats, packed executables run efficiently with no extra memory overhead or execution slowdown.
UPX is copyrighted software released under the GNU General Public License, with special exceptions that allow free use in commercial programs as outlined in the UPX License Agreement. This makes it suitable for both open-source projects and professional software distribution.
The application relies on the NRV compression library to deliver high-speed compression. An open-source alternative is available through the UCL compression library, offering compatibility for users who prefer fully open implementations.
UPX Features:
- Excellent compression ratio: typically achieves higher than WinZip, Zip, and Gzip. Use UPX to decrease the size of your distribution!
- Very fast decompression: ~10 MB/sec on an ancient Pentium 133, ~200 MB/sec on an Athlon XP 2000+.
- No memory overhead for your compressed executables because of in-place decompression.
- Safe: you can list, test, and unpack your executables. Also, checksums of both the compressed and uncompressed files are maintained internally.
- Universal: can pack several executable formats.
- Portable: UPX is written in portable endian-neutral C++.
- Extendable: thanks to its class structure, new executable formats or compression algorithms can be added easily.
- Free: Distributed with full source code under the GNU General Public License v2+. Free use in commercial programs is allowed through special exceptions stated in the License Agreement.
UPX – Pros & Cons
| Pros | Cons |
|---|---|
| ✓ Achieves strong executable compression ratios, helping reduce the size of software distributions. | ✗ It is designed specifically for executable packing, so it does not provide general-purpose file archiving features. |
| ✓ Provides fast decompression while packed applications are running. | ✗ Compression results can vary depending on the executable format and the files being packed. |
| ✓ Uses in-place decompression, avoiding additional memory overhead for compressed executables. | ✗ Its command-line and developer-focused nature may be less convenient for users looking for a graphical compression interface. |
| ✓ Maintains internal checksums and allows users to list, test, and unpack compressed executables for safer handling. | ✗ Advanced extensibility, including adding executable formats or compression algorithms, is geared toward technically experienced users. |
| ✓ Supports multiple executable formats and is portable, free, and suitable for commercial software under its license exceptions. | ✗ The provided release notes list only bug fixes, so the latest release does not introduce a notable set of new user-facing features. |
Frequently Asked Questions
Can UPX compress any EXE file?
No. It supports a range of executable formats, but not every executable can necessarily be packed successfully. Compatibility depends on the executable format and how the program was built.
Can UPX unpack an executable?
Yes. It can unpack executables that were previously compressed with UPX, allowing the original executable contents to be restored. It also provides commands for listing and testing packed files before unpacking them.
Can UPX-packed files trigger antivirus warnings?
Yes, some security software may flag packed executables because executable packing can also be used by malware authors to obscure program contents. A warning does not by itself prove that a file is malicious, so downloaded executables should be checked against a trusted source and scanned before use.
What is the difference between UPX and ASPack?
Both are executable packers designed to reduce the size of program files, but they use different packing technologies and licensing models. UPX is open source and distributed under the GNU GPL with special exceptions for commercial programs, making it a common choice for developers who want a freely available executable packer.
What’s New in UPX 5.2.1:
Release Date 2026-08-27
- bug fixes
Official Homepage – https://upx.github.io
