Reaction Time Calculator

Measure browser-observed latency for a visual prompt and summarize repeated trials.

Inputs and method

Start a trial, wait for the randomized prompt, then respond with the button or keyboard. Valid trials report best, arithmetic mean, median, and spread across all accepted trials. The retained set is limited to 100 completed trials; the five-row Recent Results section is only a view of the newest five. Early responses are discarded, and Reset All clears the complete set.

The randomized wait is scheduling time and is not included in a completed trial. A trial starts when the ready callback records its timestamp and ends when the page’s event handler records the response.

Interpreting the measurement

This measures browser-observed latency from the callback that starts the prompt until your click or key event reaches this page. Display rendering, the event loop, the operating system, and the input device can add delay. It does not isolate or measure neurological reaction time.

The browser can coarsen timer resolution, and scheduled callbacks can run later than requested. Callback execution does not establish when the changed prompt becomes visible. Do not use these values for clinical, neurological, readiness, injury-risk, or athletic-ranking decisions.

Method basis

The displayed statistics are publisher derivations from the locally retained trials. The W3C High Resolution Time specification supports use of a monotonic browser clock for elapsed-time measurement but does not guarantee sub-millisecond resolution. The WHATWG timer specification explains why callbacks can run later than requested and how a pending timeout is canceled.