Online Stopwatch

Time an activity and record laps

Select Start, then pause or resume from the same control. Record laps while running and export them as CSV. Outside form controls, Space toggles running and L records a lap.

How elapsed time stays accurate

The display refreshes through browser animation callbacks, but elapsed time comes from performance.now(), a monotonic clock designed for measuring durations. The stopwatch does not assume every callback arrives on time. If rendering pauses briefly, the next frame calculates from the original start point instead of losing those milliseconds.

Pause, resume, and laps

Pausing adds the completed segment to stored elapsed time. Resuming starts a new segment from the current monotonic timestamp. A lap records total elapsed time at that moment; it does not reset the main clock. The newest lap appears first on screen, while exported CSV lists laps in chronological order.

Worked example

Start the stopwatch, record a lap at 42.35 seconds, and pause at 1 minute 10.20 seconds. After a ten-second break, resume and record another lap at 1 minute 35.00 seconds. The paused ten seconds are excluded because the elapsed total advances only during running segments.

Tabs, sleep, and reliability

Browsers reduce rendering work in hidden tabs. The underlying elapsed calculation remains deadline-based, but the visible display may not update until the page returns. Closing or reloading the page resets the stopwatch. Device sleep, browser suspension, crashes, and operating-system clock behavior make this unsuitable for safety-critical or unattended timing.

Privacy and limitations

Times and laps remain in the page until reset or navigation. Export creates a local file. Nothing is uploaded or stored in an account. This stopwatch is useful for informal work, study, presentations, exercise, and experiments, but it is not calibrated timing equipment, a race-official system, payroll evidence, medical equipment, or a background alarm service.