How lap times reveal what a total never can
A single finish time hides everything that happened along the way. Lap splits expose the story — the fast start, the mid-session slump, the strong finish — which is why coaches and athletes obsess over them.
From workouts to cooking to study sprints, timing in segments turns a vague effort into data you can actually improve on.
How to use it
Press Start to begin timing and Stop to pause. Use Lap to record split times without stopping the clock, and Reset to clear everything. The stopwatch measures to hundredths of a second and runs entirely in your browser.
What your result means
The main display is total elapsed time since you pressed Start. Each lap you record freezes the total at that moment and shows the interval since the previous lap alongside it — the split. Reading the two together is the point: the totals tell you where you finished, the splits tell you where the time actually went.
What limits accuracy is not the clock but the hand pressing the button. The browser timer is good to a few milliseconds, while human reaction time is around 200 milliseconds, so treat the hundredths column as a tiebreaker rather than a measurement. For anything officially timed, use equipment triggered by the event rather than by a person.
Why this one is different
Every lap keeps its own row and its own time, so splits can be read back afterwards rather than remembered. The clock is driven from timestamp differences instead of counted ticks, which is why leaving the tab in the background — where browsers throttle timers to save power — does not quietly lose seconds.
How it works
Elapsed time is measured with a monotonic clock — a counter that only ever moves forward and is unaffected by the system clock being corrected, by daylight saving, or by a network time sync landing mid-run. Reading the ordinary calendar clock instead would let a background adjustment add or subtract seconds from your timing.
The display refreshes about thirty times a second, but the number shown is always the real difference between the start moment and now, not a count of refreshes. Pausing stores the accumulated total and restarts the measurement from a fresh reference point, so stopping and starting repeatedly does not accumulate error. A lap records the total at the moment you press it; the split beside it is that lap minus the one before.
Worked example
Three laps pressed during a run, reading the total each time:
Lap 2 total 01:28.10 → split 45.50
Lap 3 total 02:09.35 → split 41.25
The totals only ever rise, so pace has to be read from the splits: this runner slowed by nearly three seconds on the second lap and then finished fastest of the three. A single final time would have hidden both.
Frequently asked questions
How accurate is it?＋
It uses the browser's high-resolution clock, so it's accurate to within a few milliseconds — plenty for workouts, cooking and study sessions.
What is a lap time?＋
A lap records the total elapsed time at the moment you tap Lap, letting you compare splits without stopping the clock.
Does it keep running in a background tab?＋
Elapsed time is derived from timestamps rather than counted frames, so the reading stays correct even if the browser throttles the tab. The display refreshes as soon as the tab is visible again.
Should I trust it for race timing?＋
For training, yes. For official results, no: hand timing carries a human reaction delay of roughly 0.2 seconds at each end, which is why competition uses electronic timing.
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 time at that point. Both are useful, and pacing analysis usually needs the laps.
Can I export the lap times?＋
Copy them from the list. Nothing is uploaded, so if you need a permanent record, paste it somewhere before closing the tab.
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Assumptions & limitations
Accuracy notes:
- Timing uses the browser's high-resolution clock, which is accurate to well within the reaction time of the person pressing start.
- Browsers deliberately reduce timer precision as a security measure, so treat readings below about 10 milliseconds as indicative.
- Background tabs are throttled by the browser; elapsed time is still calculated correctly, but the display may update less often.
- Lap times are the interval since the previous lap; the split is the total elapsed time at that point.
Sources & references
Timing precision in the browser, and where it stops being enough:
- MDN Web Docs — performance.now() and deliberate precision reduction
- World Athletics — why official results use electronic timing, not hand timing