Solve
Start the stopwatch and mark a lap for each interval you want to measure.
Loading tool…
Quick guide
Understand the calculation
Stopwatch with lap recording, per-lap observations, and CSV export. It measures against the browser's monotonic clock, shows each lap's Δt next to the running total, and summarises the series with mean, sample standard deviation, minimum and maximum.
When to use this tool
Use it when the figure you are after is a repeated, comparable interval: reaction times, the endpoint of a titration, oscillation periods, fall times, or the duration of each stage of a protocol. Laps turn a single run into a series of replicates, and the per-lap observation records what happened in each one without leaving the table. If what you need is an alarm or a countdown, this is not the tool: it measures forwards and never notifies.
How it works
Enter starts and pauses; Space marks a lap. Elapsed time is recomputed against the browser clock on every frame instead of accumulating frames, so the reading never drifts. Each lap stores its Δt — the difference against the previous lap's total — alongside the running total, and takes an observation of up to 50 characters. From two laps onward the tool computes n, mean, sample standard deviation (n−1), coefficient of variation (CV = σ/μ), minimum and maximum, always over the Δt values and never over the running totals. The CSV exports the log in chronological order; the summary does not travel in the file because it is recomputed from the Δt column, and appending it would break the rectangular shape every importer expects.
The clock is the browser's monotonic one, not the system time: the measurement survives a clock adjustment on the machine, but it is not a timestamp either. On screen milliseconds are truncated and the hour field never wraps; the CSV rounds them to whole milliseconds. Dispersion is the sample form (n−1) and appears from the second lap. Nothing is stored: reloading the page discards the log, so export before you leave.
Examples
- Lap 2 marked with the clock at 00:00:25.000, after a first lap at 00:00:12.400
Δt = 25.000 − 12.400 → 00:00:12.600 - Triplicate of 12.400 s · 12.520 s · 12.310 s
μ = 00:00:12.410 · σ = 00:00:00.105 - CSV row for the first lap of that triplicate
1,12400,12400,""
Frequently asked questions
Does it keep measuring if I switch tabs?
Yes. In a background tab the browser stops repainting and the digits freeze, but the time is computed against the clock every time it is read rather than by adding up frames: when you come back, the reading jumps to the correct value with nothing lost.
Which standard deviation does it compute?
The sample one, with n−1 in the denominator, over the lap Δt values. Laps are a handful of repetitions of a measurement, not the whole population of possible times. It appears from the second lap onward.
How accurate is the last digit?
Less than it suggests. Browsers deliberately reduce the resolution of their high-precision clock for privacy, and on top of that comes the reaction time of whoever presses the key, on the order of tenths of a second. Milliseconds are useful for comparing laps with each other, not as a bound on absolute accuracy.
Are laps kept when the page reloads?
No. Everything lives in the tab's memory; reloading or closing it discards the log. Export the CSV before you go.
Does timing continue when I minimize the bubble?
Yes. When you reopen it, the time includes the minimized interval, and laps and their notes are preserved. If paused, that reading is preserved. Closing the bubble or reloading the page discards the session; export the CSV if you need to save the laps.