Prime95 Small vs Large vs Blend FFTs: Which Torture Test Preset?
Quick facts: Prime95 30.19 build 20 offers four Torture Test presets plus Custom. Smallest FFTs test the L1 and L2 caches with high power, heat and CPU stress. Small FFTs test the L1, L2 and L3 caches with maximum power, heat and CPU stress. Large FFTs stress the memory controller and RAM. Blend, the default, tests all of the above. Since version 29.8, the program sizes each preset automatically from your CPU’s cache, so there is no current fixed FFT size table to memorise.
The official definitions, without the folklore
The definitions above come from the Torture Test menu itself and the program’s stress.txt guidance. They are short on purpose: each preset is defined by where its data lives. When the FFT data fits inside the processor caches, the cores can work continuously with few pauses, which is why the small presets draw so much power and produce so much heat. When the data is too large for the caches, the processor must constantly fetch from system memory, which shifts the pressure onto the memory controller and the RAM itself. Blend mixes data sizes across that boundary, so over time it exercises both paths. Custom lets you set the minimum and maximum FFT size, the amount of memory to use, and how long to spend on each size, which is useful only when you already know which boundary you want to probe.
| Preset | Official focus | What a failure most often points to |
|---|---|---|
| Smallest FFTs | L1/L2 caches; high power, heat, CPU stress | Core voltage or frequency settings, or cooling limits |
| Small FFTs | L1/L2/L3 caches; maximum power, heat, CPU stress | Core stability and the true temperature ceiling under load |
| Large FFTs | Memory controller and RAM | Memory timings, memory voltage, or controller settings |
| Blend | All of the above; default preset | A broad mix; use the notes below to narrow it down |
| Custom | Your chosen sizes, memory amount and timing | A targeted follow-up once you know what you are hunting |
Why old FFT size tables mislead
Search results still surface tables that assign exact ranges to each preset, such as a small range for Small FFTs and a larger range for Large FFTs. Those numbers are from versions published many years ago. The version 29.8 change notes record that Torture Test options are now generated from cache size, and version 30.19 follows that approach. On a modern processor with large L3 cache, the sizes the program picks will differ from a processor with a small cache, and both can be correct. In Custom mode you can see the minimum and maximum FFT values the program offers on your own machine; treat those as your machine’s values, not as a universal specification. If a guide quotes a fixed range without naming a version, assume it is historical until proven otherwise.
Which preset should you run first?
For a new build at stock settings, start with Blend for a broad check, then follow up with the specific preset that matches any symptom. For an overclocked CPU, start with Small FFTs: because it is the maximum heat and power preset, it reveals cooling limits and core instability quickly. For a new memory kit, a changed memory profile, or random crashes that happen outside stress tests, include a long Large FFTs run, because Blend alone spends part of its time on small sizes that fit in cache and may not press memory as continuously. Many testers run all three in sequence across different sessions rather than expecting one preset to answer every question.
The most useful diagnostic clue in the menu
The Torture Test menu carries an official note worth remembering: if Blend fails but the small FFT tests pass, the problem is more likely in memory or the memory controller. Read that in reverse as well. If Small FFTs fails quickly at a given frequency, changing memory timings first is unlikely to fix a core problem. This one clue can save hours of random setting changes. Record which preset failed, which worker stopped, and how long the run lasted before you change anything. Change one variable at a time, then repeat the same preset; switching presets at the same time as settings makes the comparison meaningless.
Small FFTs, heat, and your cooler
Because Small FFTs is officially the maximum power and heat preset, it is the right test for judging a cooler, but also the test most likely to expose weak cooling. Before blaming the processor, check that the cooler is mounted evenly, thermal paste coverage is sensible, and case fans actually spin up under load. Two widely recommended large air coolers for demanding stress testing are linked here as sponsored Amazon links: the Noctua NH-D15 and the Thermalright Peerless Assassin. Sponsored: this site may earn a commission from purchases made through those links at no extra cost to you; see the affiliate disclosure. A cooler upgrade helps only if your case airflow can feed it, so treat cooler, paste, and airflow as one system. If your processor is already hitting its limit, read Prime95 overheating and 100C readings before running another long test.
A practical testing order
First, run Small FFTs long enough to see temperatures level off and to catch quick core errors. Second, run Large FFTs to press memory and the controller. Third, run Blend for the longest session, ideally following the official duration guidance of 6 to 24 hours for a final check. Between stages, save results.txt and note your settings. If every preset passes but the machine still misbehaves in games or rendering, the fault may lie in a component Prime95 does not test, such as the graphics card or storage; Prime95 stresses CPU and RAM, plus cooling and power delivery indirectly, and the official guide points to other tools for the rest.
Next steps
New to the program itself? Follow How to Use Prime95 first. For errors and shutdowns, use the troubleshooting hub. The complete tutorial index lives in the tutorials hub.