Stopwatch

How accurate is an online stopwatch?

Accurate enough for anything a human starts and stops, which is the real limit here. Your reaction time on the start and stop is somewhere around a fifth of a second in each direction, so the hundredths on the display are precise but your timing of a sprint is not. For anything where that matters, a stopwatch of any kind is the wrong instrument and you want a beam or a pad.

What this does guarantee is no drift. Elapsed time is calculated from timestamps rather than by adding up intervals, so a busy tab or a slow machine cannot cause the count to fall behind, which is a real failure of naive implementations left running for an hour.

What is the difference between a lap and a split?

A lap is the time for one segment on its own. A split is the total elapsed at the moment you marked it. Runners generally want laps, to compare one repetition against the next; anyone timing progress through a longer course usually wants splits. The table here shows both for each mark, plus which lap was fastest and slowest and the average, so the pattern in a set is visible without exporting anything.

Can I use it with the keyboard?

Space starts and pauses without needing to find the button, which is the difference between timing something and fumbling at it. The shortcut ignores keystrokes aimed at form fields and buttons, so it will not fire twice when a control already has focus.

Does it keep running if I switch tabs?

Yes, and the elapsed time shows in the tab title so you can read it from the tab strip without switching back. The screen is held awake while the stopwatch runs, so a phone will not lock partway through and lose your view of it. Closing the tab entirely does end the session, since nothing here runs on a server.