Wait for the field to change, then hit it as fast as you can. Repeat a few times and you get your best, your median and your average in milliseconds — which is a more honest picture than a single attempt, because one lucky guess is not a reaction time.
What is actually being measured
Not your reflexes alone. The number is the whole chain: the screen has to paint the change, your eye has to catch it, your brain has to decide, your hand has to move, and the mouse or touchscreen has to report the press back to the browser.
That matters when comparing results. A 120 Hz monitor with a wired mouse can read forty or fifty milliseconds faster than a phone, with exactly the same person at the other end. Compare your own attempts with each other; treat someone else’s number from a different device as a different measurement entirely.
Where the clock starts
This is the detail that separates a test worth taking from one that flatters you. Most implementations start timing when they set the variable that changes the colour — but nothing has been drawn at that point. The browser still has to lay the page out and paint it, which takes a frame or several, and none of that is time you spent reacting.
Here the clock starts inside an animation frame after the change has actually been painted, so the elapsed time begins when the signal was visible rather than when it was decided. On a result that lands around 250 ms, the difference between the two is roughly a tenth — and always in the direction that makes you look faster.
Why the wait is random, and why an early press does not count
The delay before the signal varies between roughly one and four and a half seconds. A fixed delay is learned in two or three attempts, and after that the test measures how well you can count rather than how fast you react.
Pressing before the signal is caught and called out rather than recorded. So is anything under 100 ms after it, which is faster than nerve conduction allows and therefore a guess that happened to land well. Both cost you the attempt, which is the only way to make guessing unprofitable.
Median or average
Both are shown, and they answer different questions. The average includes everything, so a single lapse of attention — a notification, a thought about something else — drags it up by a long way.
The median is the middle attempt, and it ignores that. For a handful of attempts it is the better summary of what you are typically capable of, which is why the comparison band is based on it rather than on the mean.
Typical numbers
Simple visual reaction time for adults sits somewhere around a quarter of a second, with most people landing between 200 and 350 milliseconds. Below 200 ms is genuinely quick. Above 350 ms usually says more about tiredness, the device or a touchpad than about anything else.
Reaction time is slower to a visual signal than to a sound, slower again to touch, and it lengthens gradually with age and considerably with fatigue. It is also trainable, up to a point — the first few attempts of any session tend to be the slowest, which is another reason a single try means little.
FAQ
What is a good reaction time?
For a simple visual signal, 200–250 ms is fast, 250–350 ms is the usual adult range, and under 200 ms is exceptional. Bear in mind that a fast screen and a wired mouse are worth tens of milliseconds on their own, so a number is only comparable against itself on the same device.
Why did it say I was too soon?
You pressed before the signal, or within 100 ms of it — faster than a nerve impulse can travel from eye to hand, which means the press was already on its way. Either way it was anticipation rather than reaction, so the attempt is discarded instead of being recorded as a very fast time.
Can I use the keyboard instead of clicking?
Yes. Focus the field with Tab and press Space or Enter. A key press is often marginally faster than a mouse click because the travel is shorter — one more reason to compare attempts made the same way.
Why does my phone give a slower result than my computer?
Touchscreens add latency that mice do not: the panel samples your finger at a fixed rate and the system waits briefly to see whether the touch becomes a scroll. Combined with a slower display refresh, that is easily 30–60 ms before your reflexes are involved at all.
Is my result stored or uploaded?
No. The whole test runs in your browser, nothing is transmitted, and nothing is kept once you leave the page. The copy button gives you a line of text to paste wherever you like — that is the only thing that goes anywhere, and only because you chose to.