What is Stopwatch?

Stopwatch is a centisecond-precision timer with unlimited lap splits. The display reads HH:MM:SS.CS so you can see hundredths of a second. Tap Lap to record split times without pausing the main timer, label each lap if you want, and export the whole session as CSV or TXT when you're done.

The timer uses requestAnimationFrame for smooth on-screen updates and reads time off performance.now, which means it stays accurate even if the tab loses focus for a moment. Lap rows show split time, cumulative total, and an editable label, with the fastest and slowest splits highlighted and the average split shown below. Optional sound cues confirm each start, lap, and stop hands-free, and Reset asks to confirm before clearing once laps are recorded.

How to use

1. Step 1 — Press 'Start' to begin the stopwatch. The timer counts up in hours, minutes, seconds, and milliseconds.
2. Step 2 — Press 'Lap' to record split times without stopping the main timer.
3. Step 3 — Press 'Stop' to pause the timer. Review your lap times, then reset to start fresh.

When to use

• Pacing yourself through interval workouts and reviewing how each split landed.
• Timing a rehearsed presentation section by section to fit a slot.
• Recording experiment runs in the lab when a wall clock isn't precise enough.

Result

Timing interval training at the gym: start the stopwatch, hit 'Lap' after each set of 30 seconds, then review your lap times to see if you stayed consistent across all 8 rounds.

FAQ

How precise is the timer compared to my phone's built-in stopwatch?

It reads to the hundredth of a second and updates on every animation frame. Underlying time comes from performance.now, which is sub-millisecond. Display drift over an hour is typically a few centiseconds, similar to any built-in stopwatch.

What's the difference between Lap and Stop?

Lap captures a split time and adds it to the list while the timer keeps running. Stop pauses the main clock entirely. You can press Start again after Stop to resume from the same elapsed time without losing any laps you already recorded.

Can I rename a lap after I've recorded it?

Yes. Each lap row has a label field you can click to type into. Useful for marking which set or which segment a lap belongs to, which is what shows up in the CSV export later.

What format does the CSV export use?

Three columns: lap number, lap time, and cumulative total time, plus your label as a fourth column when present. Times are written as HH:MM:SS.CS strings, so they open cleanly in spreadsheets without needing a custom number format.

Will closing the tab lose my laps?

No — your elapsed time and lap list are saved locally and come back automatically if you refresh or reopen the page, so an accidental reload won't wipe them. And once you've recorded laps, Reset asks you to confirm before clearing, so a stray tap can't erase your splits. For a permanent copy, download the CSV or TXT before you finish.

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