Stopwatch
Precise timing down to the millisecond
How does an online stopwatch work?
An online stopwatch measures elapsed time by recording the moment you press start and continuously calculating the difference from the current time. Each tick updates the display with hours, minutes, seconds, and milliseconds elapsed since that start point. When you pause, the accumulated time is stored so you can resume without losing progress.
Lap timing lets you record intermediate marks without stopping the clock. Each time you press Lap, the stopwatch captures the current elapsed time as a total and calculates the split — the time since the previous lap — by subtracting the last lap's total from the current elapsed time. This shows both how long each segment took and your cumulative time from the start.
Online stopwatches use performance.now() rather than Date.now() for accuracy. Unlike Date.now(), which reads the system clock and can jump if the clock is adjusted, performance.now() returns a monotonic timestamp that only moves forward. It also offers sub-millisecond precision, making it ideal for timing tasks where every fraction of a second counts.
Frequently asked questions
How accurate is an online stopwatch?
Clockr's stopwatch is accurate to the millisecond, updating the display every 10 milliseconds for smooth visual feedback. It uses high-resolution timing APIs that are not affected by system clock adjustments. For everyday timing — workouts, cooking, experiments — an online stopwatch is just as reliable as a physical one.
What is a lap time and how is it calculated?
A lap time records how long a single segment took. When you press Lap, Clockr stores the total elapsed time and calculates the split by subtracting the previous lap's total. For example, if your first lap total is 1:30 and your second is 3:00, the second lap split is 1:30. The display shows both the split for each segment and the running total from the start.
What is the world record for the 100 metre sprint?
Usain Bolt holds the men's world record at 9.58 seconds, set at the 2009 World Championships in Berlin. Florence Griffith-Joyner holds the women's record at 10.49 seconds, set in 1988. These times are measured to a hundredth of a second using electronic timing systems far more precise than any consumer stopwatch.
How do I time multiple people at once?
For a single shared start, begin the stopwatch when the race starts and record each finisher's time individually by pressing Lap as they cross the line — each lap captures that runner's finish time as a cumulative total. For independent starts, open separate browser tabs with a stopwatch for each person. Clockr stores up to 20 lap times per stopwatch session.
What is a split time in running?
A split time is the duration of a specific segment of a run — for example, the time for one lap of a track or one kilometre of a longer race. Splits help runners assess pacing: even splits mean each segment took the same time; negative splits mean you ran the second half faster than the first. Clockr's lap feature shows each split alongside the cumulative total.
How do professional athletes use stopwatches in training?
Coaches use stopwatches to time intervals, rest periods, and repetition splits during training sessions. Swimmers time each lap off the wall; sprinters measure acceleration phases and full-effort reps; endurance athletes track kilometre splits to monitor pacing. Consistent split data over weeks reveals whether an athlete is improving or fatiguing during a session.
What is the difference between a stopwatch and a timer?
A stopwatch counts up from zero, measuring how much time has elapsed since you started. A timer (or countdown timer) counts down from a preset duration to zero and alerts you when time is up. Use a stopwatch when you don't know how long something will take; use a timer when you need to limit an activity to a set period. Clockr offers both tools.
How many decimal places does a stopwatch measure to?
Clockr displays time to three decimal places — milliseconds — in the format HH:MM:SS.mmm. Professional sports timing often uses hundredths of a second (two decimal places), while scientific measurements may need microseconds. For virtually all everyday and fitness uses, millisecond precision is more than sufficient.