CPU Stress Test
Load every CPU core to 100% in your browser — no install. One Web Worker per core runs prime-number math while you watch live operations per second and per-thread activity, then get a score you can compare across devices.
Plug a laptop in and close heavy tabs first — battery savers and background load pull the score down.
On battery, laptops throttle the CPU long before it breaks a sweat. Plug in and pick a performance profile to see the real ceiling.
Scores only match on the same stress level, duration, and browser. Ultra is the steadiest pick for machine-vs-machine.
A readout that starts strong then slides means the CPU is dropping its clocks to cool off — thermal throttling, made visible.
A processor rarely shows its weaknesses in everyday use — it shows them under sustained load. A clogged fan, dried-out thermal paste, an aggressive power-saving profile, or a thin chassis that simply cannot shed heat all look fine at idle, then surface as throttling the moment every core runs flat out. This test loads all of them at once and holds the load, so you can hear whether the fans spin up, feel where the heat goes, and see whether the ops-per-second readout holds steady or sags as the chip pulls its clocks back. It is a quick pulse check before buying a used laptop, after repasting a CPU, or when a machine feels slower than it should — and everything runs locally in your browser.
When you press Start, the page spawns one Web Worker for every logical core the browser reports, and each worker independently grinds through prime-number checks — trial division, the same arithmetic-heavy loop native stress tools use — counting completed operations as it goes. The first second is a warm-up and is discarded, so spin-up noise never skews the result; after that, the live number is the combined throughput of all threads, sampled ten times a second. The bars underneath show each thread on its own: on modern hybrid CPUs it is normal for some bars to sit lower — those are efficiency cores doing the same work at a slower pace. The final score is the run’s average ops/sec, scaled by the stress-level multiplier.
How to read your score
| Score | Rating | What it typically means |
|---|---|---|
| 50,000+ | Elite | Recent high-end desktop CPU with many fast cores and cooling to sustain them |
| 30,000 – 50,000 | Exceptional | Modern performance desktop or a top-tier laptop chip on mains power |
| 15,000 – 30,000 | Excellent | Solid mid-range desktop or a recent performance laptop |
| 7,500 – 15,000 | Good | Mainstream laptop or an older desktop — comfortable for daily work |
| 3,500 – 7,500 | Fair | Budget, compact, or aging hardware; expect slowdowns under heavy load |
| Under 3,500 | Poor | Very old or heavily throttled — check cooling, power mode, and background load |
Choosing a stress level
| Level | Multiplier | Best for |
|---|---|---|
| Low | ×1 | A quick pulse check with minimal heat |
| Medium | ×2 | The default — a moderate load safe for laptops on battery |
| High | ×4 | A realistic sustained workload, like rendering or compiling |
| Extreme | ×8 | A thermal test: watch whether the readout sags as the chip heats up |
| Ultra | ×16 | Benchmarking — the heaviest math per operation and the most consistent scores between machines |
Is it safe to max out the CPU in a browser?
Yes. Browser JavaScript runs in a sandbox — it cannot raise voltages, change fan curves, or bypass thermal limits, so the worst it does is warm the machine, spin the fans, and drain the battery faster. Modern CPUs throttle themselves when they run hot, which is exactly what a sagging readout shows, and closing the tab stops the load instantly. Just remember a browser benchmark also measures the browser: the same machine scores differently in Chrome, Safari, and Firefox, so compare like for like. If a game stutters while your score here looks healthy, the bottleneck may be rendering rather than the processor — check that with the Frame Rate Tester.