Stopwatch Timer — Precise Time Measurement
A precise stopwatch with lap timing capability. Measure elapsed time down to centiseconds. See also Countdown Timer and Time Zone Converter.
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How the Stopwatch Timer Works
This stopwatch measures elapsed time from the moment you press Start until you press Stop. It uses high-resolution timing based on the system clock (Date.now()) and updates the display every 10 milliseconds for smooth visual feedback. The lap feature records split times, allowing you to measure individual segments while the total time continues running. The timer maintains accuracy by always calculating elapsed time relative to the start timestamp rather than accumulating intervals.
Time Measurement Formula
Elapsed Time (ms) = Current Timestamp - Start Timestamp + Accumulated Time
Hours = floor(Elapsed / 3,600,000)
Minutes = floor((Elapsed % 3,600,000) / 60,000)
Seconds = floor((Elapsed % 60,000) / 1,000)
Centiseconds = floor((Elapsed % 1,000) / 10)
Lap Time = Current Total - Previous Lap Total
Common Uses for Stopwatches
Stopwatches are essential tools for athletics (sprint times, lap times, interval training), cooking (timing recipes, fermentation), science experiments (reaction timing, process measurement), education (timed tests, presentations), productivity (Pomodoro technique, task timing), and gaming (speedrunning, time trials). The lap feature is particularly useful for tracking multiple segments of an activity, such as individual laps in a race or intervals in a workout.
Time Units Reference
| Unit | Milliseconds | Seconds | Display Format |
|---|---|---|---|
| 1 centisecond | 10 | 0.01 | 00:00:00.01 |
| 1 decisecond | 100 | 0.1 | 00:00:00.10 |
| 1 second | 1,000 | 1 | 00:00:01.00 |
| 1 minute | 60,000 | 60 | 00:01:00.00 |
| 1 hour | 3,600,000 | 3,600 | 01:00:00.00 |
| World 100m record | 9,580 | 9.58 | 00:00:09.58 |
| World mile record | 223,130 | 223.13 | 00:03:43.13 |
| Marathon world record | 7,219,000 | 7,219 | 02:00:35.00 |
Timing Accuracy
Browser-based stopwatches typically achieve accuracy within 10-16 milliseconds due to JavaScript timer resolution and browser scheduling. This is more than sufficient for most practical timing needs (sports, cooking, productivity). For scientific or professional timing requiring sub-millisecond accuracy, dedicated hardware timers with crystal oscillators are recommended. The display updates every 10ms (centisecond precision), which matches the practical accuracy of the underlying timer.
Frequently Asked Questions
How accurate is this stopwatch?
The stopwatch is accurate to approximately 10-16 milliseconds, which is the typical resolution of JavaScript timers in modern browsers. It calculates elapsed time from system timestamps rather than accumulating intervals, preventing drift over long durations.
What is the difference between lap time and split time?
Lap time is the duration of a single segment (from the previous lap mark to the current one). Split time (shown as "Total Time" in the lap table) is the cumulative time from the start. For example, if your first lap is 30 seconds and second lap is 25 seconds, the split time at lap 2 is 55 seconds.
Can I resume the stopwatch after stopping?
Yes. When you stop the timer, the elapsed time is preserved. Pressing Start again (shown as "Resume") continues from where you left off. Only the Reset button clears the accumulated time back to zero.
Will the stopwatch continue if I switch browser tabs?
Modern browsers may throttle timers in background tabs, which can affect the display update frequency. However, since the stopwatch calculates elapsed time from timestamps, the displayed time will be correct when you return to the tab — it just may not have updated smoothly while in the background.
What is the maximum time the stopwatch can measure?
The stopwatch can theoretically run for millions of hours since it uses JavaScript's number type for milliseconds. Practically, it displays up to 99:59:59.99 (nearly 100 hours) in the HH:MM:SS.ms format. For longer durations, consider using a date calculator instead.
How do I use the lap feature for interval training?
Start the stopwatch at the beginning of your workout. Press Lap at the end of each interval (e.g., each sprint, each set, each rest period). The lap table will show individual interval times and cumulative totals, plus statistics like fastest, slowest, and average lap times.
Why does the display show centiseconds instead of milliseconds?
Centiseconds (hundredths of a second) are displayed because they provide a good balance between precision and readability. Full millisecond display would change too rapidly to read, while centiseconds update at a pace that is visually trackable and matches the practical accuracy of browser-based timing.