Online Stopwatch — Free, Accurate, With Laps

The tool is more precise than you are

It displays hundredths of a second and records lap times. And the elapsed figure is computed by subtracting a stored start instant from the current clock on every frame, rather than by adding a fixed amount on an interval.

That distinction is worth explaining, because it is the difference between a stopwatch and a thing that looks like one. Intervals do not fire with perfect regularity, and browsers throttle them heavily in a hidden tab — so a timer that accumulates on each tick drifts, silently, and one left running for an hour can be out by a noticeable margin with nothing to indicate it. Reading the clock every frame makes the displayed time correct regardless of how the loop behaved.

Which means the limit is your reaction time

Even a perfect stopwatch is started and stopped by a person, and human reaction to a visual cue is on the order of a fifth of a second — far larger than the hundredth the display shows.

So the last digits of any hand-timed measurement are decoration. They are real, in that the tool computed them correctly, and they are not meaningful, in that they tell you about the moment your finger moved rather than the moment the event happened.

Two practical consequences follow:

• Take several measurements and use the median rather than trusting one. A single hand-timed figure carries your reaction time as an unknown offset; several of them let it average out.
• Do not compare two results that differ by less than your own reaction time. That difference is noise, and treating it as a finding is how people conclude one method is faster than another on no evidence.

What laps are for

Splitting a long activity into segments, so you can see where the time went rather than only the total. A twenty-minute run of work broken into four laps tells you which part was slow; the total cannot.

The useful comparison is between laps within one run, because they share the same person pressing the same button and therefore carry the same bias. Comparing a lap from today against one from last week compares two reaction times as well as two activities.

What it is good for, and what it is not

Exercise intervals, cooking, rehearsing a presentation to see whether it fits, comparing how long two approaches to a task take, and anything where a few tenths would not change the decision.

Not for anything competitive or official. Those need equipment triggered by the event itself — a beam, a pad, a gate — precisely because a hand cannot be trusted to a hundredth of a second, however good the clock behind it is.

Nothing is uploaded and no times are stored beyond the page. The distinction from the Pomodoro timer is worth naming: a stopwatch counts up and measures what happened, while a Pomodoro timer counts down and bounds what you will do. Knowing which question you are asking tells you which one to open.