RunToolRunStopwatch
Use a stopwatch to measure time.
How to Use the Stopwatch
Press start
The stopwatch counts up with millisecond precision.
Record laps
Tap lap to mark split times without stopping the clock.
Stop and review
Pause anytime and read the full list of lap times.
How a browser stopwatch stays accurate
The naive implementation adds ten milliseconds to a counter a hundred times a second. It drifts immediately, because timers in a browser are not guaranteed to fire on schedule — they wait for the main thread, which is busy doing other things.
The correct approach records the moment you pressed start and, on every update, displays the difference between then and now. Individual updates can be late without affecting accuracy, because each one recomputes from the anchor rather than accumulating. That is how this stopwatch works, and it is why the elapsed time stays correct even when the page is busy.
Background tabs and power saving
Browsers deliberately throttle timers in hidden tabs, sometimes to once per second or less, to save battery. A stopwatch that accumulates ticks loses time badly under throttling; one anchored to a timestamp does not, because the difference between start and now is unaffected by how often the display refreshed.
The visible consequence is only that the digits stop updating smoothly while the tab is in the background. Bring it forward and the correct elapsed time appears, because it was never being counted, only displayed.
What the milliseconds are worth
The display shows hundredths or thousandths of a second, but the meaningful precision is limited by something else entirely: your reaction time. Human reaction to a visual cue is roughly 200 to 250 milliseconds, and pressing start and stop by hand introduces that error twice.
So a hand-timed result is accurate to perhaps a quarter of a second regardless of how many digits are shown. This is precisely why competitive sport uses electronic starting gates and finish beams rather than a person with a button.
Laps and splits are different measurements
A lap is the time since the previous lap marker; a split is the total elapsed time at that point. Both are useful and they answer different questions — laps show consistency between segments, splits show progress against a target for the whole.
Confusing them makes pacing data meaningless. If lap three reads 1:02, that is either a one-minute segment or a total of just over a minute, and the two describe completely different performances.
Practical use
Beyond sport, the common uses are all measurement rather than competition: how long a build actually takes, how long a process runs, how long a task really occupies compared with how long it feels. The last is frequently the most useful, because estimates of one's own time are consistently wrong in the same direction.
Everything runs locally in your browser with no server and no account, so timings are not recorded anywhere. The corollary is that they are not saved either — note anything you need to keep before closing the tab.