What is reaction time? Definition and parts
The reaction time definition in one line: the time between a signal and your response to it, in milliseconds.
A clock starts when the signal appears and stops when the response is registered, which can be a press, a key or a catch. The meaning is the same on a screen, in a car or on a track. What changes is the signal, the response and the equipment that does the timing.
The definition of reaction time in psychology and sport is the same idea: how quickly a person starts to respond to a stimulus. To compare two results, write down two things for each: what the signal was and what counted as the response.
The parts of a response
- Detecting The eyes, ears or skin pick up the signal and pass it on.
- Deciding You work out what to do. In a simple task there is almost nothing to decide.
- Moving The hand, foot or finger starts, and the press or catch registers.
We do not split the milliseconds between the three parts, because a browser test cannot separate them. It times the whole chain, and your screen, input device and browser sit inside the number too.
Simple vs choice reaction time
Simple reaction time is one signal and one response, like the test above. Choice reaction time is two or more signals, each with its own response. In one study of 150 adults aged 18 to 80, on a computer keyboard and a 60 Hz screen, the group mean was 274 ms for the simple task and 475 ms for a four-choice task, with the same people on the same equipment.2 A second study gave 1,466 adults aged 18 to 65 a two-button letter choice task, and the group mean was 550 ms.3
The signal matters too. In a lab study of five trained observers, the group mean was 161 ms for a sound and 207 ms for a light.4 These are group means from particular setups. Treat them as context, not as a target, and see the average reaction time page for the full list with methods.
Reaction time examples
- Catching a dropped ruler. A partner lets go and you close your fingers. The distance the ruler fell gives the time, as the ruler drop test shows. The catch itself is part of the result.
- Braking in a car. The lights ahead come on, and the foot starts to move toward the brake. The reaction time is the gap between the two, and the car keeps moving during that gap. This site gives no driving figures and is not a driving test.
- A race start. A runner reacts to the starting signal. The F1 reaction test lets you practice pressing the moment a row of start lights goes out.
- A screen game. A shape changes color and you click. That is a simple reaction time test, and the reaction time test on the home page times it over five rounds.
- A choice task. A left arrow means press the left key, a right arrow means press the right key. You have to decide as well as react, so the time is longer than for a simple task.
How a browser test measures reaction time
The test above draws a signal on the screen and reads the time of your press from the browser's input event, then subtracts the time the signal frame was shown. Several delays sit inside that number besides you.
| Part of the chain | What is known |
|---|---|
| Screen refresh | The signal can only appear on a refresh, so up to one frame: 16.7 ms at 60 Hz, 6.9 ms at 144 Hz, 4.2 ms at 240 Hz |
| Monitor | 11.0 ms in one lab study, measured with a photodiode1 |
| Mouse | 6.8 ms for a gaming mouse in that study, and 20 ms or more for standard drivers1 |
| Keyboard | A standard USB keyboard can add 20 to 40 ms, as Bridges and colleagues state it citing earlier work5 |
The page measures none of these on your setup and subtracts none of them. So a browser reaction time is you plus your screen, your input device and your browser. It suits comparing your own runs on the same setup, and it does not suit comparing different devices or different people.
For a time with no screen in it, the ruler drop test turns the distance a ruler fell into milliseconds. That includes the catch, so its number is not the same as a screen number.
Read how this test measures for the sources of error and the rules behind each round.
Reaction time FAQ
What is the reaction time meaning in simple words?
To define reaction time simply: it is how long you take to respond after something happens. It is the gap between a signal, such as a light, a sound or a touch, and the start of your response, such as a press or a catch. It is measured in milliseconds, which are thousandths of a second.
What is the reaction time formula?
Reaction time is the time of the response minus the time of the signal. A dropped ruler has its own physics formula, t = √(2d / g) with g = 9.80665 m/s², which turns the distance the ruler fell into a time: 143 ms for 10 cm. The ruler drop test page has a calculator, a conversion table and a printable ruler.
What are reaction times, and why do they differ between tests?
Reaction times are the measured gaps between a signal and a response. They differ because the task, the person, the signal and the equipment differ. A choice task takes longer than a simple one, and a screen, an input device and a browser each add their own delay. The average reaction time page lists published figures with how each was measured.
What is the difference between simple and choice reaction time?
Simple reaction time is one signal and one response. Choice reaction time is several signals, each needing its own response, so you have to decide as well as detect and move. In one study of 150 adults the group mean was 274 ms on a simple task and 475 ms on a four-choice task, with the same people on the same equipment.
How do you measure reaction time?
With a clock that starts at a signal and stops at a response. On a screen, use the reaction time test above: five rounds, scored by the median. Without a screen, use the ruler drop test with a partner. The two methods time different things, so keep their results apart.
What is a good reaction time?
It depends on the test, so no single number answers it. The reaction time test sorts your median into bands that are ours, not a published norm, and the average reaction time page shows published figures with their methods. Your best comparison is your own earlier runs on the same setup. This is a browser game for fun, not a medical, driving or safety test.
Reaction time formula
The general reaction time formula is a subtraction: the time of the response minus the time of the signal.
reaction time = t(response) − t(signal)
In a browser test the signal time is when the screen showed the signal, and the response time is the time stamp of your press. The page does the subtraction and shows whole milliseconds.
The one physics formula that gives a reaction time without a clock is the ruler drop. A ruler released from rest falls a distance d = g t² / 2, so the time is
t = √(2d / g), g = 9.80665 m/s²
A ruler caught at 10 cm took 143 ms to fall. The ruler drop test has a calculator, a conversion table and a printable ruler. The equation assumes a free fall and ignores air drag, and the number includes your fingers closing, so it is not a screen reaction time.
Sources
- Woods DL, Wyma JM, Yund EW, Herron TJ, Reed B. Factors influencing the latency of simple reaction time. Frontiers in Human Neuroscience 9:131 (2015). doi:10.3389/fnhum.2015.00131. https://pmc.ncbi.nlm.nih.gov/articles/PMC4374455/ (checked Nov 8, 2026)
- Deary IJ, Liewald D, Nissan J. A free, easy-to-use, computer-based simple and four-choice reaction time programme: the Deary-Liewald reaction time task. Behavior Research Methods 43:258-268 (2011). doi:10.3758/s13428-010-0024-1. https://doi.org/10.3758/s13428-010-0024-1 (checked Nov 8, 2026)
- Woods DL, et al. Age-related slowing of response selection and production in a visual choice reaction time task. Frontiers in Human Neuroscience 9:193 (2015). https://pmc.ncbi.nlm.nih.gov/articles/PMC4407573/ (checked Nov 8, 2026)
- Hanson JVM, Whitaker D, Heron J. Preferential processing of tactile events under conditions of divided attention. Neuroreport 20(15):1392-1396 (2009). doi:10.1097/WNR.0b013e3283319e25. https://pmc.ncbi.nlm.nih.gov/articles/PMC3364445/ (checked Nov 8, 2026)
- Bridges D, Pitiot A, MacAskill MR, Peirce JW. The timing mega-study: comparing a range of experiment generators, both lab-based and online. PeerJ 8:e9414 (2020). doi:10.7717/peerj.9414. https://pmc.ncbi.nlm.nih.gov/articles/PMC7512138/ (checked Nov 8, 2026)
Every figure on this page was read in the source itself, not in a summary. Secondhand statements are attributed to the paper that made them. The standard gravity value, 9.80665 m/s², is the conventional value, and the ruler times are our calculation from it.