How accurate is an online stopwatch?

Three layers of timing

1. The clock. Browsers expose performance.now(), a monotonic clock that never jumps when the system time changes. For privacy reasons browsers deliberately coarsen its resolution (to between about 5 µs and 1 ms depending on browser and settings), which is still far finer than a hundredth of a second. Its long-term accuracy comes from your device's crystal oscillator; typical quartz clocks are specified to within tens of parts per million, a fraction of a second per day.

2. The display. A screen redraws 60–120 times a second, so the number you see can lag the real value by up to one frame (8–17 ms). That's why the thousandths digit on any on-screen stopwatch appears to jump rather than count. The stored lap times aren't affected, because they come from the clock at the instant you pressed, not from what was on screen.

3. You. Pressing a button in response to something takes around 0.15–0.25 s, and the delay varies from press to press. Measure yours on the reaction time test. Because both the start and the stop involve a press, what matters is how different the two delays are, typically a few hundredths up to two tenths of a second.

Why most web stopwatches drift, and this one doesn't

The naive way to build a stopwatch is to run a timer every 10 ms and add 10 to a counter. Browsers don't guarantee that timing: callbacks arrive late when the page is busy, and in background tabs Chrome, Safari and Firefox throttle them to once a second or even once a minute. The counter falls behind and never catches up.

StopwatchPro instead saves the timestamp when you press Start and computes elapsed = now − start every time it draws. How often it draws doesn't matter; each frame is independently correct. The start time is also saved to your browser's storage, so closing the tab, restarting the browser or letting the phone sleep won't lose a running time.

Laps vs splits

Both come from the same button presses. A lap (sometimes "lap split") is the duration of one segment; a split (or "cumulative split") is the total time from the start to that point. Coaches use laps to judge effort and splits to judge schedule.

How to use a stopwatch accurately

1. React to the right signal. For a race, start on the flash or smoke of the gun, not the sound: sound travels about 343 m/s, so 100 m away it arrives 0.29 s late.
2. Anticipate the finish. Watch the runner approach and press as the torso crosses the line. Waiting for them to cross and then reacting adds your full reaction time.
3. Use a physical key. A keyboard space bar is usually more consistent than a touchscreen tap.
4. Time longer efforts. A 0.2 s error is 2% of a 10 s sprint but 0.02% of a 20-minute run.
5. Report sensible precision. Hand-timed results are conventionally rounded up to the next tenth of a second in athletics. Keep the hundredths for your own comparisons.

Stopwatch vs timer

A stopwatch counts up from zero to measure how long something took. A timer counts down from a set duration and alerts you when it reaches zero. If you need countdowns, interval rounds or a Pomodoro, our sister site CountdownPro is built for that; StopwatchPro is about measuring.

Is electronic timing better?

Yes, for official results. Fully automatic timing starts from a sensor in the starting pistol and reads the finish from a photo-finish camera, removing both human presses. World Athletics requires it for records and reports it to 0.01 s. A hand-timed stopwatch is still the right tool for training, where you care about comparing your own efforts and consistency beats absolute accuracy.