How the test works
The whole gray area is one big button. Press “Start” and it changes to “Waiting…”. After a random delay of 1.5–5 seconds it turns green and says “Click now!”. Click or tap the area at that moment. The button has keyboard focus, so Space or Enter works too. The delay is chosen anew for every attempt, so you can’t count your way to the right beat.
Once you click, your “Reaction time” for that attempt is shown in ms (thousandths of a second). Press the same area again to go on to the “Next” attempt. A line at the top, such as “Attempt 3 of 5”, shows where you are.
Click before it turns green and the area goes red with “Too soon!”. That attempt is not recorded, and “Try again” repeats it with a new delay. A click less than 100 ms after it turns green is treated as a guess and handled the same way. False starts are counted next to the progress line, but there is no penalty. When you have 5 valid times, press “See results” to open “Results”.
Reading the results
If your five times were 250, 230, 310, 240 and 270 ms, the results look like this.
| Figure | Value | What it means |
|---|---|---|
| Average | 260 ms | The five times added up and divided by 5. Shown largest |
| Best | 230 ms | Your fastest time |
| Median | 250 ms | The third time when they are lined up from fastest to slowest |
| Worst | 310 ms | Your slowest time |
| Standard deviation | 28 ms | How far the times sit from the average. Smaller means more even |
Lower is faster, so “Best” is the smallest number and “Worst” is the largest. Every figure is rounded to a whole millisecond.
An average and a median that are close together mean your five attempts were even. A single stray attempt pulls them apart. Had the 310 ms above been 640 ms, the median would still be 250 ms, but the average would jump to 326 ms and the standard deviation to 158 ms. In a case like that the median is closer to how you usually do.
Under the table, “Attempt by attempt” lists the five reaction times in order. If you clicked too soon at any point, only the count is shown separately, as in “2 false starts (not counted)”, and it has no effect on the five figures above.
What affects your time
A reaction time reflects more than how focused you were at that instant. Several technical factors are mixed in.
- Display refresh rate: the most common screens redraw 60 times a second (60 Hz), and 75 Hz, 120 Hz and 144 Hz screens are also widely used. After the page asks for green to be drawn, the actual pixels change at the display’s next refresh, so a delay of up to one frame ends up in your time. That is about 16.7 ms at 60 Hz and about 8.3 ms at 120 Hz. A web page can neither measure this delay nor remove it.
- Release, not press: the test records the moment a click is completed, when you let go of the mouse button or lift your finger. The longer you hold, the longer your time, so make it a short, sharp click.
- Input device: a mouse click, a touchscreen tap and a key press each travel a different path through the operating system to reach the browser, and the delays differ slightly.
- Browser and device load: with many other tabs or background tasks running, the click can reach this page later.
- Practice and fatigue: times tend to improve a little over a few repeats as you get used to the test, and they slow down when you are tired or distracted.
For these reasons the numbers are not as precise as lab equipment or a clinical measurement. Use them to compare your own times taken on the same device with the same input method, not to measure yourself against other people or other devices.