How to Use Prime95: A Beginner’s Step-by-Step Guide
Quick facts: Current version is Prime95 30.19 build 20, a freeware program by George Woltman for GIMPS, the Great Internet Mersenne Prime Search. Its main job is hunting for Mersenne primes. The Torture Test covered in this guide is a secondary use described in the program’s own stress.txt file: it stresses your CPU and RAM by comparing every result against a known correct answer. Only the Windows version has a graphical interface. There is no installer and no administrator account is needed for normal use.
Step 1: Download the right package
Start on the download page, which lists the verified packages for version 30.19 build 20 with their exact sizes and SHA-256 checksums. Most desktop users need the Windows 64-bit package, p95v3019b20.win64.zip. Choose the 32-bit package only for a very old 32-bit Windows installation. Linux and macOS users do not get this graphical program at all; they use the command-line program called mprime, covered in separate guides linked at the end of this article. Before you run anything, you can compare the checksum of your downloaded file with the value on the download page. That simple check tells you whether the file arrived complete and unchanged from the official mersenne.org package.
Step 2: Extract to a folder you can write to
Prime95 is portable software. Right-click the zip file and extract it to a normal, writable folder, for example a new Prime95 folder inside your Documents folder or on a second drive. Avoid the default temporary location offered by some extraction wizards, and avoid protected system folders. The program needs to write its configuration and result files in the same folder where it runs, including prime.txt and results.txt. If the folder is read-only or buried in a temporary directory that gets cleaned up, settings will not save correctly and you may lose a long test’s log. Once extracted, you should see prime95.exe alongside supporting library files and text documents such as readme.txt and stress.txt. Keep all of those files together; do not move the executable on its own.
Step 3: Launch and choose Just Stress Testing
Double-click prime95.exe. You do not need to right-click and run as administrator for a normal stress test. On first launch the program may ask whether you want to join GIMPS or just run stress testing. If your goal is checking a new build, an overclock, or suspected instability, choose Just Stress Testing. That keeps the program offline from PrimeNet work assignment and puts the testing tools front and centre. Joining GIMPS is a separate, worthwhile activity in which your computer searches for Mersenne primes and reports results to the PrimeNet server, but it is not required to validate your hardware.
Step 4: Open the Torture Test and pick a preset
From the menu bar, choose Options > Torture Test. You will see preset choices named Smallest FFTs, Small FFTs, Large FFTs, and Blend, plus a Custom option. Since version 29.8, the program chooses the FFT sizes for each preset automatically from your CPU’s cache sizes, so you do not need to copy size numbers from an old forum table. Those fixed tables circulating online are from much older versions and do not describe version 30.19. Pick Small FFTs when you want maximum power, heat, and CPU stress. Pick Large FFTs when you want to stress the memory controller and RAM. Pick Blend, the default, for a broad mix of everything. The full preset comparison is in Small vs Large vs Blend FFTs.
| Preset | What it stresses | Good first use |
|---|---|---|
| Smallest FFTs | L1 and L2 caches; high power, heat and CPU stress | A quick check of core behaviour |
| Small FFTs | L1, L2 and L3 caches; maximum power, heat and CPU stress | Finding your CPU temperature ceiling |
| Large FFTs | Memory controller and RAM | Checking memory-related instability |
| Blend (default) | All of the above in a mix | A general overnight stability run |
Step 5: Watch temperatures while it runs
A stress test is only useful if you watch what your hardware is doing. Keep an eye on CPU temperature, clock speed, and voltage in a monitoring tool while Prime95 runs. Many builders use HWiNFO, a free monitoring tool available from hwinfo.com. HWiNFO is mentioned here only as a practical monitoring companion; this site has no partnership with it. There is no single official safe temperature number in Prime95’s documentation that applies to every CPU, so check the limit for your specific processor on the manufacturer’s specification page instead of relying on a generic number from a forum. If temperatures climb steadily and never level off, stop the test and improve cooling before trying again. Sudden shutdowns, a frozen screen, or a worker stopping with an error are all information, not a reason to push harder.
Step 6: Decide how long to run it
Do not judge a system by a two-minute run. The official stress.txt guide recommends 6 to 24 hours for a thorough check, notes that unstable machines often fail within minutes, and suggests short runs of 10 to 15 minutes only as a quick way to explore each step while tuning an overclock before a long final test. Running longer gives the test more chances to expose a problem; no fixed number of hours can prove a machine will never fail. The reasoning is explained in detail in How Long to Run Prime95.
Step 7: Stop the test correctly
To stop, use Test > Stop in the menu, or simply close the program after stopping the workers. Stopping preserves your log files so you can review them later. If a worker already stopped on its own with a message about rounding or a hardware failure, do not just restart and hope. Save the relevant lines from results.txt, note which worker stopped and after how long, and work through the diagnosis steps. A single error is enough to say the current settings are not stable; lowering an overclock, restoring default memory timings, or improving cooling are the usual next moves, in that order of simplicity.
What to do next
Browse the full tutorials hub for preset deep-dives, AVX settings, and platform guides. If your test produced an error, go straight to result interpretation and troubleshooting rather than repeating the same run. Keep your extracted folder, your results.txt, and a note of your BIOS settings together; that small record makes the second run far more informative than the first.
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