How Reaction Time Is Actually Measured

A reaction test looks simple. Something changes on the screen, you click, and a number appears. But that number is the sum of several steps, and not all of them happen inside you.

If you understand those steps, your score stops being a mystery. You know what you can change, what you cannot, and when a "bad" result is really just a different device.

What happens in those few hundred milliseconds

First, light from the screen reaches your eyes and the retina turns it into nerve signals. Those signals travel to the visual areas at the back of your brain, where the change is noticed. Then the brain decides to respond, sends a command down the spinal cord, and the muscles in your finger contract.

All of this usually takes around a quarter of a second for young adults in simple tests. A large study by David Woods and colleagues in 2015 measured simple visual reaction times in over a thousand people and found typical values in the range of roughly 200 to 300 milliseconds, rising slowly with age.

The part that is not you

Your equipment adds its own delay. A screen needs time to draw the new frame; at 60 frames per second a new image appears only every 16.7 milliseconds. Many screens add processing time on top. Your mouse, keyboard or touchscreen needs time to report the click, and the browser needs time to notice it.

Together these can add anywhere from about 10 to 50 milliseconds, sometimes more on phones and wireless devices. That is why the same person can score 230 ms on a gaming monitor and 290 ms on a phone. Neither number is wrong. They are measuring you plus a different machine.

Simple and choice reactions

In 1868 the Dutch scientist Franciscus Donders noticed that choosing between responses takes longer than responding to a single signal. Our Reaction Time test is a simple reaction: one signal, one response. Tests like Arrow Drop are choice reactions, where you must also pick the right key. Expect choice reaction times to be clearly slower, and expect them to grow as the number of choices grows.

How we turn many tries into one score

A single try tells you very little. You might blink, look away or guess. So we run several trials and report the median: the middle value when all your times are sorted. One very slow try barely moves the median, while it can pull an average up a lot.

We also ignore responses that come impossibly fast, under about 100 milliseconds. Those are almost always guesses, where you pressed before the signal could have reached your brain. Counting them would make your score look better than it really is.

What changes your score

Some things change it a lot: being tired, sleepy, distracted or unwell all slow reactions down. Warming up for a minute helps, because the first few tries are often slower. Your position matters too: a hand resting on the mouse, ready, beats a hand that has to travel first.

Practice helps a little and mostly makes you more consistent. Simple reaction time is fairly stable for each person, and there is no good evidence that a reaction test makes you react faster in sport or driving. It measures you on this task, today.

How to compare fairly

Compare results on the same device, with the same screen and the same input, at a similar time of day. Look at the trend over several sessions rather than one number. If you change devices, start a new baseline.

Not medical advice. A reaction test is not a medical or driving assessment. If you notice your reactions have slowed suddenly in daily life, talk to a doctor.