Stopwatch & timer
A lap stopwatch and a countdown timer with an alarm.
Stopwatch
Timer
A sound plays when it reaches 0. (when the browser tab is active)
Your phone is across the room, you are already at your desk, and you need to time a set of intervals or keep a meeting segment to five minutes. Opening a browser tab is faster than finding an app.
This page holds two independent tools: a stopwatch that records laps, and a countdown timer that plays a tone when it reaches zero.
How it works
Stopwatch
Press Start and the display counts up as MM:SS.CS — minutes, seconds, and hundredths of a second. The same button becomes Stop, and pressing Start again resumes from where you left off rather than starting over. Reset clears both the clock and the lap list.
Lap records the moment without stopping the clock. Each entry shows two numbers: the total elapsed time on the left, and the split since the previous lap on the right. Newest laps appear at the top.
Timer
Enter minutes and seconds, then press Start. The button turns into Pause, so you can hold and resume a countdown mid-run. Reset returns the display to whatever is in the input fields.
Accuracy and the alarm
Timing uses performance.now(), a monotonic clock that is unaffected by the system clock changing, so a long run does not drift the way a naive interval counter would. The alarm is a short 880 Hz tone generated by the browser's audio engine — there is no sound file to load. Because browsers throttle background tabs and block audio in some cases, keep the tab visible if the alarm matters.
Terms explained
- Lap
- A recorded time marker taken while the stopwatch keeps running. Useful for timing repeated rounds.
- Split
- The time elapsed since the previous lap, shown as the second, smaller number on each lap row.
- MM:SS.CS
- The display format — minutes, seconds, and hundredths of a second (centiseconds).
- Countdown timer
- Counts down from a duration you set and signals when it reaches zero, the opposite of a stopwatch.
- performance.now()
- A high-resolution monotonic browser clock. It only moves forward, so it is not disturbed by system time changes.
Frequently asked questions
Will the timer still run if I switch to another tab?
The countdown itself keeps tracking real elapsed time, but browsers throttle background tabs and may hold back audio, so the alarm can be late or silent. Keep the tab in front when you need the sound.
Why didn't the alarm make any noise?
Most browsers require some interaction with the page before they allow audio. Pressing Start normally counts, but if your device is muted or the tab was in the background, the tone may not play.
Can I keep my lap times after closing the page?
No. Nothing is stored, so closing or refreshing the tab clears the laps. Copy them elsewhere if you need a record.
How accurate is the stopwatch?
It reads a high-resolution monotonic clock and displays hundredths of a second. The clock itself is precise, but your own reaction time on the start and stop presses is usually the larger source of error.
Can I run the stopwatch and the timer at the same time?
Yes. They are separate and do not interfere with each other, so you can count up and count down in the same tab.
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