Stopwatch — Online Timer with Lap Times
Millisecond-precision online stopwatch with lap times and CSV export. Uses performance.now for drift-free timing.
A browser-based stopwatch that counts up in hours, minutes, seconds, and milliseconds. Record lap times with a single click, see the split between each lap and the running total, and export the lap list to CSV for a spreadsheet. Timing is based on performance.now for millisecond precision that doesn't drift.
A stopwatch is the counterpart to a countdown: instead of running down from a set value, it counts up from zero. This one has millisecond resolution and a lap feature, which makes it a good fit for workouts, speed tests, interval training, and any task where you care about the split between moments.
The display updates every animation frame using the HH:MM:SS.ms format, so you see the thousandths digit change in real time. Under the hood, timing uses performance.now (a monotonic high-resolution clock) together with an accumulator pattern: Start stores the current timestamp, Stop adds the elapsed interval to the accumulator, and Start again resumes from that accumulated value. The consequence is zero drift — the stopwatch is as accurate as your browser's timing API allows, regardless of how many times you pause and resume.
Laps are recorded with three fields: the lap number, the lap time (split since the previous lap), and the total elapsed time at the lap moment. Press the Copy CSV button to put the full lap list on your clipboard in index,lap,total format, ready to paste into Google Sheets, Excel, or any CSV-aware tool for further analysis.
- 1
Start the stopwatch
Press Start to begin counting. The display updates in real time with millisecond precision.
- 2
Record laps
Click Lap to mark a split. Each lap row shows the time since the previous lap and the total elapsed time.
- 3
Stop, reset, or export
Press Stop to pause the clock, Reset to clear both the time and the lap list, or Copy CSV to export your laps.
Workout intervals
Time sprint reps, rest periods, and circuit stations. Use the lap button to mark each round.
Performance benchmarking
Measure how long a manual task takes to calibrate realistic time estimates for future work.
Race and speed tests
Run a casual stopwatch for typing tests, speedruns, or friendly foot races with split-time tracking.
Cooking and study sprints
Time brewing, fermentation, or focused 45 minute study sessions without leaving the browser.
How accurate is the stopwatch?
Timing uses performance.now, a high-resolution monotonic clock with sub-millisecond precision. The display rounds to one millisecond. Actual wall-clock accuracy depends on your browser and CPU.
Does it keep running if I switch tabs?
The internal time continues to advance because it reads the system clock on each frame. The display may freeze in a background tab, but the elapsed time catches up when you return.
What format does Copy CSV produce?
Three columns: index, lap, and total, one row per lap. Lap and total are formatted as HH:MM:SS.ms strings, which Google Sheets parses as durations when you paste.
Can I save my laps between sessions?
Not directly. The stopwatch runs in memory and clears on refresh. Use Copy CSV to preserve your laps in a spreadsheet or document.
Is anything uploaded or tracked?
No. The stopwatch runs entirely in your browser. No timing data leaves your device.