A little help along the way

Follow these steps. Every option is explained underneath. Everything happens in your browser; nothing is sent anywhere.

Choose what to react toA colour change is the usual test. A sound, choosing a side and start lights are the others; targets, clicks a second and the spacebar counter test speed of another kind.
Choose how many triesFive tries give a quick score; ten or twenty a steadier one. Clicks are counted over 1 to 60 seconds, and targets in tens.
Press Start, then waitKeep your finger ready. The change comes after a wait that cannot be guessed. Press Space or Enter, click or tap the box the moment it comes.
Read your resultYour middle try comes first, then the average, the fastest and slowest, every try, and what your setup adds.

Every option explained

Too soon and guesses: A press before the change is too soon, and a press under 100 ms after it is a guess, faster than people react. Both are counted and the try is taken again.
Your middle try: The score is the median: the middle of your tries, so one slow or lucky try does not move it. A try over a second is left out.
How it is timed: The change is made in an animation frame and timed by that frame; your press is timed by its own event time. A beep is scheduled on the audio clock and placed on the page's clock.
What no page can see: Your keyboard, mouse and screen add their own delay, often 10 to 70 ms, and touch screens more. No web page can measure or take that away, so compare results only on the same device.
Your setup: Each result says how finely this browser reads its clock, how often the screen is drawn and what you pressed with. A browser that rounds its clock to a screen frame gets no score.
Leaving the page: A try stops when the page loses focus or is hidden, and is taken again.

Reaction time test