Stopwatch
Time anything to the hundredth of a second and record laps, with each lap time, the running total and the fastest, slowest and average lap.
Updated Formula covered by automated tests
Press Start to time anything from a 100-meter sprint to a two-hour exam, Stop to pause, and Lap to record a split without stopping the clock. Each lap row shows the time for that lap alone and the running total at the moment you pressed the button, and once you have two or more laps the tool reports the fastest, slowest and average lap.
Runners use it for interval sessions on a track, swimmers for repeats, coaches for timing drills, teachers for timed reading or science experiments, cooks for steps that need watching, and speakers for rehearsing a talk section by section. The display shows hundredths of a second, which is finer than human reaction time but useful for seeing how consistent a set of laps was.
The formula
- banked time
- Total of all earlier running periods, saved each time you press Stop
- now
- A monotonic clock reading from the browser (performance.now), which does not jump when the computer’s wall clock is adjusted
- split n
- The cumulative elapsed time at the moment of the nth Lap press
- Lap n
- Duration of the nth lap on its own
The display does not count ticks. Browsers slow down timers in background tabs and can skip frames, so a stopwatch that added 10 ms per tick would drift. Instead, every refresh recomputes the elapsed time from the stored start reading, so even if the screen updates late the number is right when it does.
Each Lap press stores the cumulative time (the split), and lap times are worked out by subtracting consecutive splits. Storing splits rather than individual laps means rounding never accumulates: the laps always add up exactly to the final total. Times are truncated, not rounded, so 0.999 s shows as 00:00.99, as on a physical stopwatch.
Worked example
A coach times a runner over three 400-meter laps of a track and presses Lap at 1:32.40, 3:06.10 and 4:41.85.
- 1 Lap 1 = 1:32.40 − 0 = 1:32.40 (92.40 s).
- 2 Lap 2 = 3:06.10 − 1:32.40 = 186.10 − 92.40 = 93.70 s, or 1:33.70.
- 3 Lap 3 = 4:41.85 − 3:06.10 = 281.85 − 186.10 = 95.75 s, or 1:35.75.
- 4 Average lap = 281.85 ÷ 3 = 93.95 s, or 1:33.95.
- 5 Spread: slowest minus fastest = 95.75 − 92.40 = 3.35 s.
Result: The runner slowed by about 1.3 s and then 2.05 s per lap, averaging 1:33.95 per 400 m. The 1,200 m total was 4:41.85.
Before you rely on the result
- Laps and times are not saved; reloading the page or closing the tab resets the stopwatch.
- Not suitable for official competition timing, which requires certified equipment.
Frequently asked questions
What is the difference between a lap time and a split time?
A split is the total time on the clock when you pressed Lap, measured from the start. A lap is the time since the previous press. In the table, the Lap Time column is each segment and the Split column is the running total.
How accurate is a browser stopwatch?
The clock itself is precise to well under a millisecond, but the result is only as good as your button presses. Human reaction time to a visual signal is typically around 0.2 seconds, and starting and stopping by hand adds that uncertainty twice, which is why official races use automatic timing.
Will it keep counting if I switch tabs or lock my phone?
On a computer, switching tabs is fine: the time is recomputed from the start reading when you return. Mobile browsers may suspend a page that is in the background for a long time or reload it to save memory, which would lose the session, so keep the page open for anything important.
Sources
- NIST Time and Frequency Division (nist.gov)