Online Stopwatch Utility (with Lap Timer)

Online stopwatch with lap splits (what this tool does)

This page is a simple online stopwatch that measures elapsed time and optionally records laps. It’s designed for everyday timing tasks—workout intervals, cooking steps, study sessions, meetings, presentations, and quick experiments—where you want a clear running time and a readable log of checkpoints.

• Elapsed time display: shows hh:mm:ss.t (hours, minutes, seconds, tenths).
• Start/Stop: start timing, pause, and resume without losing accumulated time.
• Lap logging: capture the current time as a lap, plus a split (time since the previous lap).
• Optional lap labels: add a short description to each lap (e.g., “Warm-up”, “Set 1”, “Slide 5”).

How to use the stopwatch (step by step)

1. Press Start to begin timing. The button changes to Stop.
2. Press Stop to pause. Press Start again to resume from where you left off.
3. (Optional) Type a note in Lap label (for example, “Round 1” or “Boil”).
4. Press Lap to save a checkpoint. You’ll see a table containing: lap number, label, elapsed time, and split time.
5. Press Reset to clear the timer and remove all laps.

Understanding the results: elapsed vs. split

Each lap record includes two time values:

• Elapsed: total time since you first pressed Start (minus any time paused).
• Split: time since the previous lap (the difference between this lap’s elapsed time and the last lap’s elapsed time).

Worked example (interval training)

Suppose you’re doing 3 intervals with short rests and want to label phases:

1. Press Start.
2. At the end of your warm-up, type “Warm-up” and press Lap at 00:05:30.0.
3. After Interval 1, type “Interval 1” and press Lap at 00:07:00.0.
4. After Rest 1, type “Rest 1” and press Lap at 00:08:00.0.

Your lap table would look like this:

The math behind the display (milliseconds → hh:mm:ss.t)

Internally, the timer tracks elapsed time in milliseconds and converts that number into a formatted string.

Let $\mathrm{\backslash Delta}t$ be elapsed time in milliseconds. Then:

The final formatted display is: hh:mm:ss.t.

Accuracy, assumptions, and limitations

This tool is convenient and typically quite stable for personal use, but it is not a certified timing instrument. The results depend on your device and browser.

• Browser scheduling & throttling: when a tab is in the background, browsers may throttle timers/animation frames. The display may update less often, and the lap button may feel less responsive.
• Refresh rate affects smoothness: the display updates with screen repaints, so you may see slightly different visual “smoothness” across 60/90/120 Hz screens. (The underlying elapsed time is still computed from timestamps.)
• System load: heavy CPU usage (video calls, games, many tabs) can delay updates and user input handling.
• Not for official competitions/compliance: do not rely on this tool for officiating sports events, legal/compliance timing, medical use, or any high-stakes measurement.
• Data persistence: laps exist only in the current page session. Reloading the page clears the lap list.
• Precision shown vs. precision measured: the UI shows tenths of a second. Even if the internal clock is higher resolution, your ability to click (human reaction time) dominates for manual laps.

Frequently asked questions (FAQs)

Does the stopwatch keep running if I switch tabs?

Elapsed time is computed from high-resolution timestamps, but the display updates may be throttled in background tabs. For best results, keep the tab visible.

Can I name laps?

Yes. Type a label (optional) and press Lap. If you leave it blank, the tool uses “Lap 1”, “Lap 2”, etc.

What’s the difference between “Elapsed” and “Split”?

Elapsed is total time since start; Split is time since the previous lap.

Can I export laps?

This version displays laps in a table on the page. It does not include a download/export feature.

Tips

• Use short, consistent labels (e.g., “Set 1”, “Set 2”, “Rest”).
• If you need cleaner splits, press Lap first and type labels after (or keep the label empty and rely on lap numbers).
• For presentations, consider recording a lap at each slide section to estimate pacing.