About this tool

A CPU stress test loads every processor core with sustained, real computation and watches how the system holds up — not just whether it survives, but whether throughput stays steady, whether the browser tab keeps responding, and how much work actually gets done per second. This tool spins up one Web Worker per active logical core, each running an independent trial-division primality search, and reports live operations-per-second plus a per-thread activity view so you can see the parallelism happening in real time, not just a single aggregate number.

Multi-threading matters here because a single-core test tells you almost nothing about how a modern CPU actually behaves under real workloads — video encoding, compilation, gaming background tasks, and most demanding software spread work across many cores at once. A CPU that looks strong on one thread can still bottleneck badly if it can't sustain high throughput across all of them simultaneously, and a stress test that only exercises one core will never surface that.

Key features

• One Web Worker per logical core, detected via navigator.hardwareConcurrency — every stress level drives every core
• Five stress levels (Low through Ultra) that scale how much redundant work each core does per operation, not how many cores are used
• Live ops/sec, total ops, elapsed/remaining time, and a per-thread activity view updating in real time
• Preset durations (15s / 30s / 60s) plus an Unlimited mode that runs until you click Stop
• Open-ended final score with a qualitative rating band, plus JSON/TXT export of the result

Why use it

• Get a quick read on sustained multi-core throughput without installing native benchmarking software
• Compare relative performance across your own devices, or before/after a driver, BIOS, or cooling change
• Watch for a tab that stutters or a browser that struggles to stay responsive under full-core load

Why CPU stress testing matters

Thermal throttling, unstable overclocks, and power-limit issues rarely show up during light everyday use — they surface under sustained full-core load, when a CPU has to hold its boost clocks for more than a few seconds at a time. A quick browser-based check like this one won't replace a dedicated diagnostic tool, but a throughput graph that drops steadily over a 60-second run is a useful early signal that something is worth investigating further, whether that's dust in a heatsink, an aging thermal paste job, or a power plan that's capping performance more than expected.

Tips for accurate results

• Close other CPU-heavy tabs and background applications before running a test — anything else competing for the same cores will drag your ops/sec down.
• Run Extreme or Ultra for at least 30–60 seconds if you're checking for thermal throttling; a steady or slightly declining ops/sec curve is normal, a sharp drop partway through is the signal to watch for.
• If you're comparing two machines, use the same stress level and duration on both — ops/sec is not directly comparable across different stress levels.
• Laptops should be on a hard, ventilated surface, not a bed or your lap, for a fair reading.

A quick safety note

Browser tabs are sandboxed and throttled by the operating system, so this test cannot push a CPU anywhere near as hard as a dedicated native tool like Prime95 — that's expected, and it's a large part of why this is safe to run casually. Even so, sustained high load for several minutes can noticeably warm up a laptop chassis. Keep sessions to a reasonable length, avoid leaving an Unlimited run going unattended for hours, and stop the test immediately if your device starts feeling uncomfortably hot to the touch.

Frequently asked questions