Prime95 FATAL ERROR: Rounding – What It Means and How to Fix It
Short answer: When Prime95 reports FATAL ERROR: Rounding was 0.5, expected less than 0.4 (the exact numbers vary), it means the processor produced a result that does not match the known correct result for that calculation. The Torture Test works by comparing every computation against a known correct answer, so a rounding error is the test doing its job: it found that your hardware calculated wrong under load. In almost all cases this points to instability – an overclock that is not fully stable, memory that is not fully stable, insufficient voltage for the chosen settings, or heat – rather than a fault in the program itself. Start from the troubleshooting hub if you are seeing several different symptoms at once.
What the numbers in the message actually mean
Prime95 knows what the correct result of each FFT calculation should be, within a small expected margin of rounding error. Normal floating point arithmetic is not infinitely precise, so a tiny rounding difference is expected and allowed. The error appears when the observed rounding is larger than the allowed margin: in plain terms, the answer came back too far from correct to be explained by normal rounding. That is why the message prints two numbers, what was observed and what was expected as the limit. A single wrong bit in a long calculation is enough to push the result outside that margin. The worker that hit the error stops, writes the failure to results.txt, and the other workers may keep running, which is why you should read the full log rather than only the last line on screen. Our guide to reading Prime95 results explains how results.txt is structured.
Common causes, in the order to suspect them
| Cause | Why it produces a rounding error | Quick check |
|---|---|---|
| Overclock or undervolt that is not fully stable | Raised clocks or lowered voltage leave less timing and electrical margin, so a demanding FFT calculation can return a wrong result even while games and desktop work seem fine. | Return clock and voltage controls to their default or Auto settings and retest. |
| Memory instability | An unstable memory overclock, tight timings, or a marginal kit can corrupt data before or during the calculation. | Disable the memory overclock profile (for example XMP or EXPO) and retest at the base memory speed. |
| Heat under sustained load | As temperature rises over a long run, marginal settings that passed a short test can start to fail. | Watch temperatures while the test runs and note whether the error appears only after the system has fully warmed up. |
| A marginal core, voltage setting or BIOS setting | One core, one voltage rail or one aggressive BIOS option can be the weak point rather than the whole system. | Change one setting at a time and retest; see the worker clues below. |
Use the worker number to locate the problem
Each worker in Prime95 tests an assigned slice of work, and on a modern processor each worker is associated with particular threads. When only one worker reports the rounding error and stops while the others continue, that pattern matters: it suggests the instability is tied to a particular core or thread path, a per-core overclock setting, or the voltage delivered under that specific load, rather than a total system failure. Write down which worker failed, how long it ran before failing, and which FFT size was running at the time. If the same worker fails early and repeatedly at stock settings, that is stronger evidence than a single late failure. Our guide to a worker that stopped in Prime95 covers this isolation process in more depth.
Step-by-step troubleshooting order
- Save the evidence first. Copy the error line, the worker number, the preset (Small FFTs, Large FFTs or Blend) and the elapsed time from results.txt before you change anything.
- Return the whole system to default settings. Load optimised defaults in the BIOS or firmware, remove CPU and memory overclocks, and run the same preset again. If the error disappears at stock settings, the hardware is most likely fine at stock and your previous settings were not fully stable.
- Isolate CPU from memory with the presets. Small FFTs keep data in the CPU caches and put maximum power, heat and CPU stress on the processor, by the program’s own definition. Large FFTs stress the memory controller and RAM. Blend tests everything. The Torture Test menu itself notes that if Blend fails but Small FFTs passes, the problem is often in memory or the memory controller. Test one preset at a time so the result tells you which subsystem to investigate.
- Check temperature while the test runs. A monitoring tool alongside Prime95 shows whether temperatures climb steadily until the failure. If errors arrive only when hot, work through the cooling checks in our overheating guide before changing electrical settings.
- Change one electrical setting at a time, conservatively. If you choose to re-apply an overclock, restore it in small steps, staying within the limits in your CPU and motherboard documentation, and retest after each step. Never treat a voltage copied from a forum post for a different processor as a safe value for yours.
- Retest for a meaningful length. A five minute pass does not undo a rounding error. The official stress.txt guide recommends 6 to 24 hours for a thorough check, and notes that unstable systems often fail within minutes, while others only fail after hours.
What not to do
Do not ignore the error and keep using the same settings for important work. A machine that calculates wrong under load can corrupt files, saves and calculations silently; the error you saw is the visible version of a problem that does not always announce itself. Do not raise several voltages at once to make the error go away, because you will not know which change mattered, and excessive voltage creates its own risks. Do not assume the program is buggy: according to stress.txt, you should suspect the hardware first, and only consider a software problem if the exact same error repeats at the exact same place every time. Finally, do not declare the system fixed after one short run; instability that took an hour to appear needs a longer run to rule out.
Next steps
If the error persists at fully default settings with reasonable temperatures, work through the broader decision tree in Hardware failure detected in Prime95, and read Prime95 errors vs an unstable system to judge how serious a single error is for your use case. Both guides stay within the same rule: change one variable at a time, keep notes, and let longer tests have the final say.