Tap to start, wait for the colour to change, and tap again as fast as you can. Your time is shown in milliseconds.
Most people react in around 200 to 300 milliseconds to a visual cue. Tiredness, distraction and a slow screen or connection can all add to that, so try a few times and take your best.
Tapping before the colour changes counts as a false start, so wait for the signal rather than guessing.
Frequently asked questions
What is a good reaction time?
Around 200 to 250 milliseconds is typical for a visual reaction test.
Why do my results vary?
Attention, sleep and device lag all change the result. Take several attempts.
Is a keyboard or a screen tap faster?
It varies by device. Try both and compare.