How the reaction time test works
After a random wait of 1.5 to 5 seconds the test area turns green, or a beep plays. Press as soon as you notice it: click, tap, or press Space or Enter. Your reaction time is the gap between the stimulus and your press. A round is 5 attempts, and your score is their average; the median is shown too, because it ignores a single slow attempt.
Pressing before the stimulus is an early start. So is a response under 100 ms, which is faster than human reaction and means you guessed. Early starts are retried rather than counted, and so is an attempt with no press within 5 seconds.
How it's timed
The green isn't drawn by the page's usual rendering. The test shows it directly inside the display frame it belongs to, and the timer starts at that frame's timestamp. Your press is timed with the timestamp the browser gave it when your system delivered it, so a busy page can't add to your result.
The beep is scheduled on the audio hardware's clock rather than started by a timer, and its time is mapped to the page's clock at the moment your audio device plays it, including the output latency your browser reports.
What your result includes
No web page can see the light leave your screen or the sound leave your speakers. Every result includes some hardware delay: up to one display refresh plus the monitor's processing, your mouse or keyboard's own latency, and for the beep any audio latency your browser doesn't know about, such as Bluetooth. That's a constant few to a few tens of milliseconds, so compare results taken on the same setup.
Switching tabs or windows during an attempt discards it: browsers slow down pages you aren't looking at.
Questions
What is a good reaction time?
For a visual stimulus on a typical setup, most people average between 200 and 300 ms, and trained players often score 150 to 200 ms. Reactions to sound are usually 30 to 50 ms faster than reactions to light. Your hardware adds its own delay, so results from different setups are not directly comparable.
Why is my reaction time slower here than in another test?
Tests differ in how they time the stimulus. Many start the clock when the page asks for the color change, which can be a frame or more before it appears, and time your click when the page gets around to handling it. This test times the stimulus to the frame that draws it and your press to the moment your system delivered it. Different monitors, mice and browsers also change the number.
Does my monitor refresh rate affect my score?
Yes. At 60 Hz a new image appears every 16.7 ms, at 144 Hz every 6.9 ms, so a faster display shows the green sooner. Monitor processing adds more, from a few milliseconds on gaming monitors to 30 ms or more on some TVs outside game mode.
Why does a very fast result count as a false start?
Responses under 100 ms are too fast to be a reaction to what you saw or heard: the eye, brain and hand need longer than that. A response that quick means you started moving before the stimulus appeared, so it is treated as an early start and retried.
Should I use a mouse or the keyboard?
Use whichever you react with fastest and stick with it, since devices add different amounts of delay. Some keyboards add 10 ms or more of latency, while gaming mice are often under 5 ms. Tapping a touchscreen works too.
Can I improve my reaction time?
Practice helps up to a point, mostly by making you more consistent. Sleep, alertness, and warming up matter more than most people expect. Focus on the center of the test area and keep your finger resting lightly on the button.
Why does the audio version ask me to avoid Bluetooth?
Bluetooth audio buffers sound before playing it, which adds 100 to 300 ms, and browsers often cannot tell how much. Wired headphones or built-in speakers keep the beep timing accurate.