Prime95 puts a heavy calculation load on your CPU so you can check temperatures and look for calculation errors. It can help test cooling and stability, but it does not repair hardware, measure gaming performance or prove that every part of the PC is working perfectly.
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Download Prime95 from the official site
Prime95 is free. Use the GIMPS download page and choose the build for your operating system. On Windows, extract the ZIP into a folder, then open Prime95. Check the included readme because options can change between releases.
If offered a choice, select Just Stress Testing for a local test. You do not need to join the distributed prime-number search or set the program to start whenever Windows boots.
Before you start
- Save your work and close other demanding programs.
- Check that the CPU cooler and fans are fitted and working.
- Open a temperature-monitoring tool supported by your hardware.
- Look up the temperature limits for your exact CPU; there is no single safe number for every processor.
- For troubleshooting, start with known stock settings rather than adding an overclock.
Not sure which processor or drivers you have? Save a report with our DxDiag guide. Check the motherboard‘s own support list as well; a chipset name alone does not show every supported feature.
Which Prime95 torture test should you choose?
| Test type | Useful starting point | Limit |
|---|---|---|
| Small FFT options | A heavy CPU/cache workload for observing cooling and power behaviour. | Heat depends on CPU, instructions and selected settings. |
| Blend | A broader CPU and memory workload. | An error does not identify the faulty component by itself. |
| Large FFT or Custom | A different data footprint or a repeatable investigation. | Not a replacement for every dedicated RAM test. |
In the desktop app, look for Options > Torture Test. Labels and AVX options vary by build. Keep a note of the chosen settings so later results are comparable.
How to test CPU cooling
- Record the idle temperature and room conditions.
- Start a suitable CPU-focused test and watch it closely at first.
- Check temperature, fan behaviour, clock speeds and any thermal-throttling indicator together.
- Stop the test if temperatures reach the processor’s specified limit, cooling fails, or the computer behaves abnormally.
- Use the app’s stop control, usually Test > Stop, when finished.
A successful short run is only an initial check. Longer runs may find problems missed early on, but leaving an overheating PC running for hours is not a better test.
What do errors and high memory use mean?
A worker error, freeze or restart shows that this workload did not complete reliably. Possible causes include settings, cooling, memory, power or hardware faults. Change one thing at a time and repeat the same test. Do not assume that an error proves the CPU is damaged.
Blend can intentionally use a lot of memory. If you need to understand Windows memory categories first, see our RAMMap explanation. Passing Prime95 also leaves GPU, storage and everyday application behaviour to check separately.