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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →To run a CPU stress test with Prime95, download the official archive, extract it, launch prime95.exe on Windows (or ./mprime -m on Linux/FreeBSD), choose Just Stress Testing, and start a Small FFTs torture test. Small FFTs is the quickest way to expose CPU-core, cache, voltage and cooling instability. Follow it with Blend when you need a broader CPU, memory-controller and RAM stability check. Watch temperatures and hardware-error logs throughout; stop immediately if the processor reaches its documented thermal limit, throttles abnormally, crashes or reports calculation errors.
What Prime95 tests
Prime95 is the client used by the Great Internet Mersenne Prime Search (GIMPS), but its torture-test mode is also a demanding stability diagnostic. It repeatedly performs calculations and compares the results with known-correct values. A mismatch means the computer did not complete that calculation reliably.
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The workload can stress CPU execution units, L1/L2 cache, the memory controller, system memory, CPU and case cooling, motherboard power delivery and the power supply. It is therefore inaccurate to call Prime95 a CPU-only test. The official stress-test notes list overheating, DIMMs, inadequate cooling, power-supply problems, processor settings, memory timings and overclocking among possible causes of incorrect calculations or crashes. See the official Prime95 stress-test documentation.
Which Prime95 version should you download?
The official GIMPS download page lists Prime95 30.19 build 20 as the latest stable release, dated June 2, 2024. It offers separate 64-bit and 32-bit Windows ZIP archives. The same server lists a newer 31.04 build 5 64-bit Windows preview dated March 5, 2026, but that preview is in the version directory rather than on the main download page. The 31.05 directory contains source code only, not a Windows executable. Use the stable 30.19 build 20 for a general-purpose test unless you specifically need to investigate the preview.
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| Release | Status in the official listing | Best choice for a normal stability test |
|---|---|---|
| 30.19 build 20 | Latest stable release on the main download page; June 2, 2024 | Yes |
| 31.04 build 5 (Windows 64-bit) | Preview; March 5, 2026; listed in the version directory | Only when you intentionally want to test a preview |
| 31.05 | Source code directory; no Windows executable listed | No Windows binary to use |
Download the archive from GIMPS Prime95 downloads. Prime95 is portable: there is no conventional installer. Extract the ZIP to a folder you can write to, then run the executable.
Prepare the computer before stressing it
Prime95 deliberately creates a severe, sustained load. Preparation prevents a test from turning into avoidable data loss or thermal damage.
- Save work and close applications that cannot tolerate a crash.
- Confirm that the CPU cooler is firmly mounted and that every required fan or pump is operating.
- Open a monitoring utility that can show CPU temperature, package power, clock speed and thermal throttling. Also plan to check Windows hardware-error logs (for example, WHEA events) after the run.
- Know the processor manufacturer’s documented maximum temperature and the system’s normal thermal-protection behavior. There is no universal Prime95 cutoff: a fixed rule such as “85°C is always safe” or “95°C always damages every CPU” is not technically valid across generations.
- Do not leave an unfamiliar, recently overclocked, undervolted or newly rebuilt system unattended during its first test.
Stop if the processor reaches its specified thermal limit, throttles unexpectedly, powers off, or continues rising to an unsafe temperature. A cooler temperature target may be appropriate for your particular case, motherboard or noise limit, but it should not be presented as a universal Prime95 rule.
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1. Download and extract the archive
- Open the official GIMPS download page.
- Choose the Windows 64-bit ZIP for a normal modern 64-bit installation, or the 32-bit ZIP when you are running 32-bit Windows.
- Extract the ZIP. Prime95 can run from the extracted folder and does not need an installer.
2. Select stress-testing mode at first launch
- Run
prime95.exe. - At the first-use prompt, select Just Stress Testing. This keeps Prime95 from requesting work assignments from GIMPS.
- If the torture-test window does not open automatically, choose Options → Torture Test.
3. Choose the workload and threads
- Select a torture-test mode. For a CPU-focused first pass, choose Small FFTs.
- Set the number of CPU cores to test. For a full-load check, select all available physical cores.
- If the processor supports simultaneous multithreading (called Hyper-Threading on some CPUs), enable Use hyperthreading for the full logical-thread load.
- Click OK and watch the system for the first several minutes.
Current Prime95 releases use a number of cores field plus a separate Use hyperthreading checkbox. Guides that tell you to enter a total number of torture-test threads describe an older interface; the dialog changed in Prime95 30.7. The change is documented in the Prime95 release notes.
Choose the right torture-test mode
| Mode | What it emphasizes | When to use it | Important limitation |
|---|---|---|---|
| Small FFTs | CPU cores and cache with working data that largely remains in processor cache; typically the fastest way to create a very high CPU load | First test for core, cache, voltage, cooling and overclock/undervolt stability | It still exercises memory, cooling and power-delivery behavior; it is not literally CPU-only |
| In-place large FFTs | High power and heat with larger data sets and greater pressure on the memory controller and RAM | When you need a hotter, broader load than Small FFTs | It is not a dedicated memory diagnostic |
| Blend | A changing mix of FFT sizes covering CPU/cache and larger-memory workloads | General system-stability validation after the CPU-focused run | It is not a RAM-only test and does not replace a dedicated memory test |
| Custom | User-selected FFT range and memory allocation | A specific diagnostic plan, such as targeting a particular cache or memory behavior | Values must match the fault you are investigating; there is no single universally correct custom setting |
For a straightforward CPU stress test, start with Small FFTs. After that passes, use Blend to look for instability that appears only when larger memory regions and the memory controller are involved. The Prime95 guide describes how Custom settings can target different FFT and memory ranges.
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- 20 cores (8 P-cores plus 12 E-cores) and 28 threads. Discrete graphics required
- Up to 5.6 GHz with Turbo Boost Max Technology 3.0 gives you smooth game play, high frame rates, and rapid responsiveness
- Compatible with Intel 600-series (with potential BIOS update) or 700-series chipset-based motherboards
- DDR4 and DDR5 platform support cuts your load times and gives you the space to run the most demanding games
Set cores and hyperthreading deliberately
A full-load validation normally uses every physical core and enables Use hyperthreading when the CPU supports it. This produces the greatest sustained logical-thread load and is the most relevant configuration for a workstation or gaming PC that may run heavily threaded software.
For troubleshooting, repeat the same workload with hyperthreading disabled. If the computer passes with hyperthreading off but fails when it is on, the result indicates instability associated with the additional logical-thread load, but it does not identify a single defective part. CPU voltage, the memory controller, RAM, motherboard power delivery and cooling can all be involved. Change one setting at a time and record the mode, core count, temperatures and elapsed time for each run.
How long should Prime95 run?
The official stress.txt guidance recommends 6 to 24 hours. Unstable systems often fail within minutes, while marginal faults may appear only after several hours. No duration proves that a computer is universally stable under every application and ambient temperature.
| Stage | Suggested duration | Purpose |
|---|---|---|
| Initial check | 10–15 minutes | Confirm that temperatures, clocks and power behavior are normal and that no immediate worker error appears |
| Short stability check | 1–2 hours | Catch common thermal, voltage and configuration problems before committing to an overnight run |
| Thorough validation | 6–24 hours | Appropriate after an overclock, undervolt, memory-timing change, cooler installation or system rebuild |
Run the longer test only after the initial temperature check looks safe. Ambient temperature, fan curves and the exact Prime95 version affect results, so record those conditions when comparing runs.
Monitor the run and stop it safely
What to watch
- CPU temperature and whether it approaches the manufacturer’s maximum.
- Package power, effective clock speed and signs of thermal or electrical throttling.
- Prime95 worker windows for calculation errors or stopped workers.
- Operating-system hardware logs for corrected CPU machine-check or WHEA events, even when Prime95 itself continues.
- Unexpected fan or pump behavior, freezes, display loss, reboots or power-off events.
How to stop
Prime95’s torture test is continuous. On Windows, use Test → Stop, then exit Prime95 normally. Do not kill the process first unless the system is unresponsive or is becoming thermally unsafe; in that situation, use the operating system’s emergency shutdown procedure after protecting the hardware.
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On Linux and FreeBSD, press Ctrl+C to end a command-line torture test.
What counts as a failed test?
Record the run as a failure if any of these occurs:
- A worker reports
FATAL ERROR. - A worker reports a rounding error, calculation mismatch or another incorrect result.
- Prime95 says that a worker stopped unexpectedly.
- The operating system freezes, crashes, shows a blue screen, reboots or powers off.
- Hardware-corrected errors appear in system logs, including CPU machine-check or WHEA events.
- The processor reaches its documented thermal limit or throttles severely enough that the test is no longer representative of the intended configuration.
A Prime95 mismatch means the system failed to perform that workload reliably; it does not, by itself, prove that the CPU is defective. Cooling, DIMMs, memory timings, voltage settings, motherboard power delivery, the power supply and other components can produce the same symptom.
Interpret the result and troubleshoot the cause
Prime95 fails within minutes
Check temperature first, then return the CPU and memory to known-good settings. Common causes include excessive temperature, insufficient or unstable voltage, an aggressive overclock or undervolt, unstable memory settings, inadequate power delivery and defective hardware. If the identical error appears at exactly the same point every time, the official documentation says that a Prime95 software defect is a rare possibility; otherwise, treat a calculation mismatch as a hardware or stability problem.
- Stop the test and let the system cool.
- Remove the overclock or undervolt and load the board’s documented default settings.
- Retest Small FFTs, then test memory separately with a dedicated memory diagnostic if Blend remains problematic.
- Check cooler mounting, fan operation, power connectors and power-supply capacity.
- Update motherboard firmware or revert a recent firmware change if the failure began immediately afterward.
Small FFTs passes but Blend fails
This pattern points more strongly toward RAM, memory timings, the memory controller or memory-related voltage settings than toward CPU cores alone. It is not conclusive because Blend also changes heat and power behavior. Reduce memory overclocking, verify the memory profile and retest; then use a dedicated memory diagnostic to isolate RAM rather than treating Prime95 Blend as a RAM-only test.
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- Intel Optane Memory Supported
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Blend passes but Small FFTs fails
Suspect CPU-core or cache instability, voltage, cooling or power delivery. Blend’s pass does not replace a CPU-focused Small FFTs run. Check temperatures and clocks under Small FFTs, then test at conservative CPU settings.
Prime95 passes but another application crashes
Prime95 does not cover every subsystem. Its official documentation notes that separate tools are needed for graphics cards, PCIe devices, disk access, networking and other components, and that the torture test cannot be considered a hard-drive stress test. A Prime95 pass therefore narrows the investigation but does not certify the entire PC.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Run Prime95 on Linux or FreeBSD
After extracting the appropriate archive and making the program executable, launch the menu interface with:
./mprime -mChoose the stress-testing option rather than joining GIMPS, then select the torture-test mode and thread settings. The official current download instructions document -m, so it is the preferred command for general instructions. Historical release notes also document:
./mprime -t-t starts the torture test directly in versions that support that behavior, but it bypasses the menu and is less suitable when you need to confirm settings interactively. Press Ctrl+C to stop a command-line torture test.
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Common Prime95 myths to avoid
“Prime95 only tests the CPU.”
Incorrect. The load also exercises cache, memory behavior, cooling and power delivery.
“Blend is a dedicated RAM test.”
Incorrect. Blend mixes FFT sizes and larger-memory workloads. Use a dedicated memory diagnostic when you need RAM-specific coverage.
“A Prime95 error proves the CPU is bad.”
Incorrect. Memory, cooling, power supply, motherboard settings and overclocking can all cause errors.
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“A stock-system failure means Prime95 must be buggy.”
Incorrect. Hidden cooling or hardware faults can exist in a new system. A repeatable identical failure at the same point is the unusual case that may justify investigating a software defect.
“Every system must run exactly 24 hours.”
Incorrect. The official recommendation is 6–24 hours, and no fixed duration establishes universal stability.
“The current dialog asks for torture-test threads.”
Outdated for current releases. Prime95 now asks for the number of cores and separately offers Use hyperthreading.
Quick Recap
A practical test checklist
- Download stable Prime95 30.19 build 20 unless you deliberately need the 31.04 preview.
- Extract the ZIP and launch the portable executable.
- Select Just Stress Testing.
- Open Options → Torture Test if the dialog is not shown.
- Run all physical cores with Use hyperthreading enabled for a full-load test.
- Start with Small FFTs and monitor temperature, power, clocks and throttling.
- Use Blend afterward for broader CPU, memory-controller and RAM-related stability.
- Stop at the manufacturer’s thermal limit, on severe throttling, or at any crash or hardware error.
- Classify a worker error, calculation mismatch, corrected hardware error or system crash as a failed run, then troubleshoot one variable at a time.
- Use dedicated tests for graphics, storage, networking and memory-specific diagnosis because Prime95 does not cover those subsystems completely.
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