Free CPU Stress Test - WebWorker Load & Benchmark in Browser

CPU Stress Test

Spawn up to navigator.hardwareConcurrency WebWorkers running a tight SHA-256 hashing loop. Each worker reports its ops/sec and the main thread sums them into a total. Adjust thread count and duration to stress-test thermals or just benchmark a machine — no install, runs entirely in your browser.

Stress settings

Threads

Duration (s)

Warning: this test intentionally maxes your CPU. Expect fans to spin up and the machine to get warm. Laptops on battery may thermal-throttle within 30-60 seconds, which is itself a useful datapoint.

Live results

—Logical cores

0Total ops/sec

0Avg per thread

0Elapsed (s)

0Peak ops/sec

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CPU Stress Test guide

How to use the CPU Stress Test accurately

The test spawns a configurable number of WebWorkers (defaulting to navigator.hardwareConcurrency, which matches your logical core count). Each worker runs a tight while loop that feeds the previous hash into a new crypto.subtle.

01 How The CPU Stress Test Works The test spawns a configurable number of WebWorkers (defaulting to navigator.hardwareConcurrency, which matches your logical core count). Each worker runs a tight while loop that feeds the previous hash into a new crypto.subtle.

02 What The Test Measures (And What It Doesn't) This tool measures sustained integer throughput across multiple cores. It is a good proxy for workloads like compilation, compression, video encode, and 3D rendering — all of which are multi-threaded and compute-bound.

03 Spotting Thermal And Power Throttling The single most informative pattern in this test is a full-speed burst followed by a sustained drop. A thin-and-light laptop will typically hold peak ops/sec for 20-40 seconds, then step down 30-50% as the CPU hits its thermal...

04 Hybrid CPUs And Per-Thread Variance On Intel 12th-gen and later, Apple Silicon (M1/M2/M3/M4), and various mobile SoCs, not all cores are equal. "Performance" cores (Intel P-cores, Apple P-cores) run at 3-5 GHz and handle heavy threads; "efficiency" cores (E-cores)...

cpu stress test browser cpu benchmark webworker stress test cpu load test cpu burn-in test sha-256 benchmark

The single most informative pattern in this test is a full-speed burst followed by a sustained drop. A thin-and-light laptop will typically hold peak ops/sec for 20-40 seconds, then step down 30-50% as the CPU hits its thermal envelope.

CPU Stress Test FAQ

Common cpu stress test questions

Is it safe to run a browser CPU stress test?

Yes on stock hardware. Modern CPUs throttle their clock speed before temperature damage, and the OS will kill the browser tab before the system freezes. If your laptop gets uncomfortably hot, stop the test. Never disable thermal protection in BIOS/UEFI for a browser benchmark.

Why does my per-thread rate vary?

On hybrid CPUs (Intel 12th-gen+, Apple Silicon, mobile SoCs) not all cores run at the same clock speed. Performance cores hit 3-5 GHz; efficiency cores run at 2-3 GHz. Spawning one worker per logical core will show performance cores at roughly 2x the ops/sec of efficiency cores, by design.

How does this compare to Cinebench or Geekbench?

It does not compare directly. Native benchmarks use hand-tuned SIMD and compiler-optimized loops that are much faster than JavaScript SubtleCrypto. The numbers here are only meaningful across runs on the same machine and browser, which is exactly what you need to spot thermal throttling or a regression.

Why does the test use SHA-256?

SHA-256 is hardware-accelerated on every modern CPU via SHA-NI (Intel) or dedicated instructions on ARM/Apple Silicon. It saturates the execution units without being bottlenecked by memory bandwidth, and it produces output that cannot be cached or optimized away, giving a predictable compute-bound load.

Checklist

Utility checks to confirm

How The CPU Stress Test Works The test spawns a configurable number of WebWorkers (defaulting to navigator.hardwareConcurrency, which matches your logical core count).
What The Test Measures (And What It Doesn't) This tool measures sustained integer throughput across multiple cores. It is a good proxy for workloads like compilation, compression, video encode, and 3D rendering — all of...
Spotting Thermal And Power Throttling The single most informative pattern in this test is a full-speed burst followed by a sustained drop. A thin-and-light laptop will typically hold peak ops/sec for 20-40 seconds...