Stopwatch

A precise online stopwatch with lap times. Start, pause and resume without losing accuracy, record up to 50 laps, and read hundredths of a second on a steady tabular display.

Reviewed by the CalcCafe editorial team · Last updated 18 July 2026 · How we test our tools

Example

Timing interval sprints at the track: press Start as the runner leaves the line, tap Lap each time they pass you, and Pause when the set ends. If they cross at 1:23.45, that is 1 minute, 23 seconds and 45 hundredths. The lap list keeps every split in order, so after four 400 m repeats you can read all four times back without scribbling anything down. Press Start again to resume exactly where you paused — the elapsed time is recomputed from the clock, so nothing is lost — and Reset clears the display and the laps for the next athlete.

How it works

When you press Start, the stopwatch records a reference timestamp (the current time minus any elapsed time already on the clock). It then refreshes the display about 20 times per second, each time computing elapsed = now − reference. Because the elapsed figure is always derived from the system clock rather than by adding up timer ticks, jitter in the refresh interval and pause/resume cycles never accumulate error. Lap stores the current reading in a list (up to 50 entries) without stopping the clock; Reset clears both the time and the laps.

Good to know

The display shows minutes, seconds and hundredths of a second (centiseconds) — the same resolution as a classic handheld sports stopwatch. Hundredths are a sensible limit for a browser tool: human reaction time when pressing a button is around 150–250 ms, so finer digits would imply a precision your thumb cannot deliver. For hand-timed events, treat results as accurate to roughly a tenth of a second.

There is a design detail worth knowing: JavaScript's setInterval is not metronome-accurate — the browser may fire it a few milliseconds late, or much later in a busy or backgrounded tab. A naive stopwatch that adds a fixed amount per tick would drift badly. This one never counts ticks; every refresh recomputes the elapsed time as the difference between the current clock time and the moment you pressed Start. The interval only controls how often the display repaints, so even if ticks arrive late, the time shown is still correct.

Lap and split are different measurements. A lap time is the duration of one segment (this circuit took 1:58.30), while a split is the cumulative time from the start (the runner passed 800 m at 4:02.11). This stopwatch records cumulative splits, which is what you want for race checkpoints; subtract consecutive entries to get individual lap durations.

Typical uses: interval workouts and circuit training, timing rests between sets, steeping tea or timing a boiling egg, speedcubing practice, board-game turns, and rough benchmarking of anything you can see start and finish. One caveat applies to all browser timers: inactive tabs are throttled to save power. The math here means the total stays correct when you come back, but the display will only catch up once the tab is visible again — so keep the tab in view if you need to watch the digits live.

Frequently asked questions

People also ask

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• Pomodoro Timer
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Sources & references

• Wikipedia — Stopwatch
• timeanddate.com — online stopwatch reference

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