How is reaction time measured?

The short answer

Reaction time is measured as the interval between stimulus onset and response registration. What a reaction time test actually measures is the entire stimulus-to-response chain: retinal transduction of light into neural signals, conduction along the visual pathway to the brain, a decision stage, the motor command, and finally muscle activation in the finger.

That makes it a measure of processing speed, not reflexes - a reflex is an involuntary spinal-loop response that bypasses the brain entirely, while a reaction requires the brain to register, decide, and act. The precision of the measurement itself depends heavily on the hardware and software used.

The data

Laboratory measurement uses dedicated hardware timers and response boxes with roughly 1ms precision, paired with tightly controlled displays. A browser test relies on performance.now(), which is itself highly accurate, but the screen's refresh cycle and the mouse or keyboard's input polling sit outside its control - together, display and input latency typically add 20-50ms to every recorded score.

What it means for you

On the same device, that added latency is roughly constant, so your scores are directly comparable with each other - real improvements and off days show through clearly. Across devices they are not: a phone touchscreen, a 60Hz office monitor, and a 144Hz gaming setup will give the same person noticeably different averages. Use online results for trend-tracking and for relative comparison against percentile tables by age, gender, and device, not for direct comparison to clinical studies.