Stopwatch

A precise online stopwatch with lap timing and lap analysis. Start, stop and record laps with the buttons or the keyboard (Space to start and stop, L for a lap), and the stopwatch lists every lap and cumulative split, highlights the fastest and slowest, and charts the laps so you can see whether your pace held up. Enter the distance of each lap, such as 400 meters on a track, to get your average pace and speed.

The time is measured against your device clock to the hundredth of a second, so it stays accurate even if the tab is in the background.

Lap time = split − previous split

Consistency = standard deviation of lap times ÷ average lap

A split is the total time when you press Lap; a lap time is the time since the previous press. The consistency figure, known in statistics as the coefficient of variation, shows how much laps vary as a percentage of the average lap: under about 2% means very even pacing. Pace per kilometer divides the total time by the total distance.

Four 400 m laps recorded at splits of 1:30.0, 3:02.0, 4:32.0 and 6:06.0 give lap times of 1:30.0, 1:32.0, 1:30.0, 1:34.0. The average lap is 1:31.5, consistency is ±2.1%, and the 1,600 m pace is 3:48 per kilometer, or 15.7 km/h. The last lap was the slowest, a sign of fading at the end.

• Track intervals, swimming lengths, rowing pieces and cycling laps.
• Timing presentations, speeches and rehearsals.
• Science experiments and classroom activities that need reaction or event times.
• Cooking, board games, puzzles and quiz rounds at parties.
• Measuring how long routine tasks really take, so you can plan your day more realistically.

The clock itself is precise to well under a millisecond, but human reaction time limits any hand-timed result. Studies of hand timing in athletics found hand-held times are typically faster than electronic times by roughly 0.1 to 0.2 seconds, mostly because timers react late to the start signal but anticipate the finish. That is why official sprint records require fully automatic timing. For training and everyday timing, a hand-operated stopwatch is more than accurate enough.

Lap timing is the backbone of interval workouts. For repeats such as 6 × 400 m, press Lap at the end of each fast repeat and again at the end of each recovery, then compare the fast laps in the table. A good session has fast laps within a second or two of each other; if the last ones slow sharply, the target pace was probably too ambitious. Swimmers can do the same per length or per 100, and cyclists per circuit. Over weeks, saving your lap lists shows whether you can hold the same pace for longer or run the same repeats faster.

Use the keyboard shortcuts for fast, repeatable laps; pressing a physical key is quicker than finding a button on screen. On a phone, keep the screen on, because locking it can pause the page. Copy the results to paste your full lap list into a training log or spreadsheet. The online timer offers a countdown mode with an alarm if you need to time a fixed period instead.