Chronometer
Millisecond stopwatch (start/pause/resume/reset/lap) plus a HH:MM:SS countdown with an alert and beep at zero.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
No laps yet — press Lap to record one.
What this tool does
- A countdown you can start without installing anything — type 00:05:00 and go; at zero the display highlights and a beep plays.
- Measure how long an operation takes: the stopwatch is millisecond accurate, and you can pause and resume it.
- Use Lap for split timing — a run or a multi-step task records each split next to the cumulative total, ready to copy out when you finish.
- Duration input is forgiving: 00:05:00, 5:00 and plain seconds all work, and the +1 minute buttons let you fine-tune without retyping.
Example
Input
Countdown set to 00:05:00; stopwatch has one lap recorded at 00:01:30
Output
The countdown reads 00:05:00 and shows 00:03:30 left after 1 minute 30 seconds; the stopwatch reads 01:30.000 and the lap table lists split 01:30.000 with total 00:01:30
Under an hour the stopwatch shows MM:SS.mmm and only adds the hour field beyond that, while the countdown always stays in HH:MM:SS. Timing advances on requestAnimationFrame, so a backgrounded tab is throttled by the browser.
Frequently asked questions
What duration formats does the countdown accept?
HH:MM:SS, MM:SS and plain seconds. Only the first segment may be any non-negative integer; the later ones must be below 60, so 1:60 is rejected while 1:59 is fine. Every segment must be numeric — an empty or alphabetic value reports an invalid duration.
Does timing stay accurate if I switch tabs?
It computes elapsed time from performance.now() differences, so throttling in a background tab only slows the repaints and the numbers snap back once you return. A closed or long-suspended tab cannot resume the current run, and the countdown keeps a fallback setTimeout so it still fires at zero.
Can I pause and continue?
Yes. Pausing stores the elapsed time and Resume continues from there, while Reset clears the stopwatch and its laps. Reset also drops the countdown duration, so you re-enter it or use the quick-add buttons.
Does Lap record the total or the split?
Both. Each lap stores the cumulative time and derives the difference from the previous lap as the split, so denser presses mean shorter splits. If the system clock jumps backwards the lap is rejected instead of showing a negative duration.
Do I need any permission for the beep?
No. The alert is a short local browser sound — no microphone access, no network call — and its loudness follows the system media volume.
Keywords：stopwatchtimercountdownchronometerlap秒表计时器倒计时计次毫秒