Online stopwatch

Yes. The time is calculated from the clock each time it is shown, so a hidden tab or a minimised window does not slow it down: when you come back, the digits are right at once. A computer going to sleep or a phone locking its screen can pause the page, so keep the screen on while timing. While the stopwatch runs, the page asks the browser to keep the screen on where that is supported.

The reading is shown to the hundredth of a second and is the difference between two readings of the browser’s high-resolution clock, so it does not drift with the screen’s refresh rate. Browsers round that clock slightly for privacy, to about a millisecond or finer. The biggest uncertainty is human: the moment someone presses the button or a key.

Press Lap or L. Each row shows the lap number, the lap time and the total time, newest on top; once there are two different laps, the fastest is green and the slowest red. “Copy laps” puts them on the clipboard as text, and “Download CSV” saves a table for a spreadsheet, with semicolons in the Russian, Spanish and French editions. Laps disappear on reset or reload.

Press F for full screen on the projector and choose white or dark. Space starts and stops, L records a lap, R resets after a stop. To time a race, start once and press L as each runner finishes: the total in each row is that runner’s time. In an experiment, a lap marks each reading, such as a pendulum swing or a falling object.