Open a simple reaction test, wait for a randomly timed go signal and respond as soon as it appears. Modern browsers can timestamp events with high-resolution timers, but the result still includes the monitor, input hardware and operating system.
For personal tracking, that limitation is manageable: keep using the same machine and compare yourself with your own previous sessions.
A classic low-tech method has one person hold a ruler vertically while another places their fingers near the bottom without touching it. The holder releases the ruler unpredictably and the participant catches it. The distance fallen can be converted into an estimated reaction time using free-fall physics.
The ruler method removes monitor and mouse latency but adds other sources of variation: how the ruler is released, where fingers start, how distance is read and whether the participant anticipates the drop.
One trial is not enough for a stable estimate. Reaction time naturally varies and people often improve during the first few attempts as they learn the test.
For casual testing, a short practice block followed by roughly 5–10 recorded trials is a reasonable personal routine. Research reliability requirements can be higher depending on the task and the precision needed.
A home reaction test cannot diagnose neurological disease, vision problems, hearing problems, concussion, sleep disorders or fitness to drive. It is a narrow sensorimotor benchmark.
If you have a medical concern about new or worsening reaction problems, use appropriate professional medical assessment rather than an entertainment website.
A browser simple-reaction test is the easiest because it calculates the timing automatically.
It can provide a rough physical benchmark, but release technique and distance reading introduce measurement error.
For casual personal tracking, use several practice trials plus a small recorded set such as 5–10 trials. Research-grade reliability may require more.