Prime95 FAQ
Straight answers to the questions people ask most about Prime95 and CPU stress testing. Every answer below follows the program’s own documentation (readme.txt and stress.txt) and the verified facts collected for this site. This is an independent resource site, not the official GIMPS website.
What is Prime95?
Prime95 is a freeware program by George Woltman, created for GIMPS, the Great Internet Mersenne Prime Search. Its main purpose is finding Mersenne prime numbers. Its built-in Torture Test is widely used to stress test a CPU and RAM by comparing computed results against known correct results.
Is Prime95 free?
Yes. Prime95 is freeware. The license file included with the program describes it as freeware provided as-is, without warranty. Prime95 is not open source software, even though a separate source package is published.
How long should I run Prime95?
The official stress.txt guide recommends running the Torture Test for 6 to 24 hours for a thorough stability check, and notes that unstable machines often fail within minutes. When overclocking, the same guide suggests short runs of 10 to 15 minutes to explore each frequency step before a long final test. There is no official promise that any fixed run length makes a system completely stable or safe.
How do I choose between Small FFTs, Large FFTs and Blend?
Small FFTs test the L1, L2 and L3 caches and produce maximum power, heat and CPU stress. Large FFTs use data sets too large for the cache, so they stress the memory controller and RAM. Blend, the default, tests all of the above. Since version 29.8 the Torture Test sizes its FFT ranges automatically from your CPU cache size.
Does Prime95 test the GPU?
No. Prime95 stresses the CPU and RAM. It does not test the graphics card. The official stress.txt guide points users to other tools, such as OCCT or FurMark, for components that Prime95 cannot test.
Is there a Mac or Linux GUI version of Prime95?
No. Only the Windows version has a graphical interface. On Linux and macOS the same code runs as a command-line program called mprime, started with ./mprime -m. The macOS build is Intel x86_64 only, so Apple Silicon Macs need Rosetta 2 translation.
What does ‘Hardware failure detected’ mean?
It means a computed result did not match the known correct result, so the program stopped that worker. According to stress.txt, an error like this usually points to a hardware problem, such as an unstable overclock, memory errors or overheating, rather than a software bug, unless the exact same error repeats at the exact same place every time.
Is Prime95 open source?
No. Prime95 is freeware, not open source software. The license included with the program provides it as-is without warranty, and the full legal terms are published by GIMPS on mersenne.org.
Keep exploring
Ready to run the test? Get the current release, Prime95 30.19 build 20, from the download page, then follow the step-by-step guides in the tutorials hub. If a test stops with an error, start with the troubleshooting hub.