A stopwatch is the simplest timing tool there is, and still one of the most useful: for laps at the track, speeches and presentations, kitchen tasks, board-game turns or a quick lab experiment. This online stopwatch measures to the hundredth of a second, records laps with split times, highlights your fastest and slowest laps and keeps running even if you reload the page.
How to use the stopwatch
- Press Start (or the Space bar) to begin timing.
- Press Lap (or L) at the end of each lap. The current lap’s time is shown under the main display.
- Press Stop (or Space) to pause. Resume continues from the same time.
- When stopped, press Reset (or R) to clear the time and laps.
- Use Copy laps (CSV) to export the lap table.
The receipt tape summarizes the number of laps and the fastest, slowest and average lap, and the table under the stopwatch lists every lap, newest first, with its split and its difference from the average.
How lap and split times are calculated
The stopwatch records the elapsed time at each press of Lap. Those are the split times. Each lap time is the difference between consecutive splits:
Elapsed time is measured with the browser’s performance.now() clock, which counts steadily from page load and never jumps when the computer’s clock is adjusted. The display is redrawn on every screen refresh, but the time itself is always calculated from the start moment, so it does not drift if the tab is in the background.
Worked example
A runner times three laps of a track:
| Lap | Lap time | Split | vs. average |
|---|---|---|---|
| 1 | 1:51.40 | 1:51.40 | +0.15 s |
| 2 | 1:49.10 | 3:40.50 | −2.15 s |
| 3 | 1:53.25 | 5:33.75 | +2.00 s |
The splits add up: 1:51.40 + 1:49.10 = 3:40.50, and adding 1:53.25 gives 5:33.75. The average lap is 5:33.75 ÷ 3 = 1:51.25, lap 2 is the fastest and lap 3 the slowest. To turn lap times into a pace per mile or kilometer, use the running pace calculator.
Getting the most accurate times
- Hand timing has limits. Human reaction time adds roughly 0.1 to 0.3 seconds at each press. That is why track and field records require fully automatic timing to the hundredth of a second, and why hand-timed results are reported only to the tenth.
- Press consistently. Using the keyboard is usually quicker than moving a mouse. Start and stop with the same finger for the most consistent results.
- Display precision is not accuracy. The display shows hundredths, truncated like a physical stopwatch, so 1:23.459 shows as 1:23.45.
Stopwatch or countdown?
A stopwatch counts up from zero and is ideal when you do not know how long something will take. To count down to a specific date and time, such as an event or deadline, use the countdown timer. To add up several recorded durations afterward, paste them into the hours and minutes calculator or the time calculator.
Frequently asked questions
What is the difference between a lap time and a split time?
A lap time is the length of one lap on its own. A split time is the total elapsed time from the start to the end of that lap. If laps take 1:51 and 1:01, the splits are 1:51 and 2:52.
What are the keyboard shortcuts?
Space starts and stops the stopwatch, L records a lap and R resets it when it is stopped. Shortcuts work while the stopwatch is on screen and you are not typing in a form field.
How accurate is it?
It reads the browser's high-resolution monotonic clock, which is unaffected by changes to the system time, and shows hundredths of a second. Any real-world error comes from reaction time when pressing the button, typically 0.1 to 0.3 seconds.
What happens if I reload the page or close the tab?
A running stopwatch keeps going: the start moment and laps are saved in this browser, and the elapsed time is restored when the page opens again. Nothing is sent to a server.
Can I save my lap times?
Yes. Copy laps (CSV) puts every lap number, lap time and split time on your clipboard, ready to paste into a spreadsheet.