How to use the online stopwatch
- Press Start (or the Space bar) to begin timing. The display shows hours, minutes, seconds and hundredths:
hh:mm:ss.cc. - Press Lap (or L) each time a lap, round or segment ends. A new row appears in the Laps table with its lap time and split time.
- Press Pause (or Space) to stop the clock, then Start again to carry on from the same time.
- Press Reset (or R) to set the stopwatch back to zero and clear the laps.
- Use Export CSV to download the laps as a spreadsheet file, and Full screen to show large digits on a projector, TV or a phone propped up on a desk.
How the stopwatch keeps accurate time
Simple web stopwatches often add 10 milliseconds every time a timer fires. That drifts, because browsers fire timers late on a busy page and throttle background tabs to about once per second or less. This stopwatch never counts ticks. When you press Start it records a timestamp, and every time the screen refreshes it calculates elapsed time = time accumulated before the last pause + (now − start timestamp), using the browser's high-resolution clock (performance.now()) with the system clock as a cross-check. A late or skipped refresh can only delay the drawing, never the measurement, so switching tabs or locking the screen does not lose time.
The start timestamp, the time accumulated so far and the laps are also saved in your browser's local storage. Reload the page or close the tab by mistake and the stopwatch picks up exactly where it was, running or paused.
The display resolution is one hundredth of a second, and times are truncated rather than rounded, as on a handheld sports stopwatch. In practice the limit is not the clock but the person pressing the button: human reaction time to a visual signal is commonly measured at roughly 0.2 to 0.3 seconds, so manual timing is fine for training, cooking or rehearsals, but not for official race results, which use automatic electronic timing.
Lap time and split time
A lap time is the duration of one segment: the time between two presses of Lap (the first lap starts at zero). A split time is the total time from the start to the moment you pressed Lap. The last split always equals the sum of all lap times. Once you have at least two laps, the fastest and slowest laps are highlighted and labelled, which makes pacing problems easy to spot.
| Lap | Lap time | Split time |
|---|---|---|
| 1 | 00:01:32.40 | 00:01:32.40 |
| 2 | 00:01:29.85 | 00:03:02.25 |
| 3 | 00:01:35.10 | 00:04:37.35 |
Here lap 2 is the fastest and lap 3 the slowest. The CSV export lists each lap time and split time as text and in milliseconds, ready for a spreadsheet.
What people use it for
- Running, swimming and cycling: lap each length or kilometre and compare your pacing.
- Presentations: rehearse a talk and lap each section to see where time goes.
- Classrooms and experiments: time a pendulum or a reading task in full screen so everyone can see.
- Work and study: measure how long a task really takes. For a fixed amount of time, use the online countdown timer instead, which also has a Pomodoro mode.
- Games and quizzes: time rounds, then pick the next player with the spin the wheel name picker or run a live team quiz.
Privacy: everything runs in your browser. The stopwatch state and laps are stored only in this browser's local storage so they survive a reload; nothing is sent to a server. More small helpers are collected on the everyday tools page.
Frequently asked questions
Does the stopwatch keep running if I switch tabs or close the page?
Yes. Time is calculated from a saved start timestamp, so a background tab, a locked screen or a sleeping laptop does not stop or slow it. If you close the tab and open the page again in the same browser, the stopwatch continues from the saved time. Clearing your browser data or using a different browser starts from zero.
How accurate is an online stopwatch?
The stopwatch reads your device's clock instead of counting timer ticks, so it does not drift or slow down in a background tab, and it shows hundredths of a second. The real uncertainty is your reaction time when pressing the buttons, typically a few tenths of a second, which is the same for any manual stopwatch.
What is the difference between a lap time and a split time?
Lap time is how long one lap took on its own. Split time is the total elapsed time at the end of that lap, measured from the start. For example, laps of 1:32 and 1:30 give splits of 1:32 and 3:02.
Can I use the stopwatch in full screen?
Yes. Press Full screen for large digits on a projector or whiteboard, and Exit full screen or Esc to return. The keyboard shortcuts keep working.
How do I save or export my lap times?
Press Export CSV. The file opens in Excel, Google Sheets, Numbers or LibreOffice Calc and has one row per lap with the lap time and split time.
What are the keyboard shortcuts?
Space starts and pauses, L records a lap and R resets. The tab title shows the running time.
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