Stopwatch
00:00.00
| Lap | Lap time | vs previous | Total (split) |
|---|
Keyboard shortcuts
- Space or S
- Start, stop, resume
- L or Enter
- Lap
- R or Esc
- Reset (when stopped)
- F
- Full screen
A key press is more consistent than a mouse click or a tap, because your finger is already on the key. Holding a key down does not repeat the action.
Laps and splits
The two words are often mixed up. A lap time is the length of one lap on its own; a split is the total time from the start to the end of that lap. The lap table shows both, plus how much faster (−) or slower (+) each lap was than the one before. In this example of four laps, the splits are what the main display would read at each press:
| Lap | Split (total) | Lap time | vs previous |
|---|---|---|---|
| 1 | 01:01.24 | 01:01.24 | |
| 2 (slowest) | 02:04.98 | 01:03.74 | +00:02.50 |
| 3 | 03:06.31 | 01:01.33 | −00:02.41 |
| 4 (fastest) | 04:06.05 | 00:59.74 | −00:01.59 |
Each lap time is the difference between two splits: lap 2 is 02:04.98 − 01:01.24 = 01:03.74. The mean lap is the last split divided by the number of laps, 04:06.05 ÷ 4 = 01:01.51. Because every lap is computed from recorded splits, rounding never adds up: the laps always sum exactly to the total.
How precise is it?
The clock. The stopwatch reads the browser’s monotonic clock, which counts in fractions of a millisecond and never jumps when the computer adjusts its time-of-day clock. It stores the moment you pressed Start and computes the elapsed time from it, so it cannot drift, fall behind in a background tab or lose time on a slow computer.
The press. Many web stopwatches read the clock when their script gets round to handling a click, which can be tens of milliseconds late on a busy page. This one uses the time stamp the browser attaches to the key press or click itself, so a slow frame does not shift your result.
The display. Most screens redraw about 60 times a second, roughly every 17 ms, so the last digit of a millisecond display changes faster than any screen can show. The digits you see while it runs are a preview; the time recorded at each press is exact, and it is what you see when you stop and in the lap table.
The person. The largest error in hand timing is reaction time: a fraction of a second between seeing the event and pressing the key, which differs a little each time. When you start and stop on events you react to in the same way, much of it cancels out. For results decided by hundredths, use electronic timing such as light gates; for everything else, hundredths are honest.
It keeps running
The stopwatch saves its state in your browser’s local storage every time you press something: the start moment, the time already counted and the laps. Reload the page, close the tab or restart the browser, and it carries on from the right time; laps are kept until you press Reset. Your times and laps are never sent to a server, and the page works without a connection once it has loaded. The tab title shows the running time, so you can follow it from another tab.
For a countdown, use the timer; to be told when the clock reaches a time, the alarm clock; to work out the time between two clock times without measuring it, the time calculator.