Stopwatch › Guides › How accurate is an online stopwatch?
Why a browser stopwatch should measure elapsed time from the clock rather than count timer ticks, and why your reaction time is the biggest source of error.
A browser timer such as setInterval is not guaranteed to fire on time. MDN documents that timers can run late, and that browsers throttle timers in background tabs to save power. A stopwatch that simply adds 10 ms per tick will therefore drift, and can lose a lot of time when the tab is hidden.
Our stopwatch reads the device clock when you press start and shows the difference from that moment. The display refreshes every few milliseconds, but the elapsed time does not depend on how often it refreshes, so a hidden tab does not make it lose time.
When you press start and stop by hand, your reaction time adds to the result. That is why official sports timing uses automatic systems. For workouts, cooking, study sessions and everyday timing, a browser stopwatch is more than precise enough.
Yes. Elapsed time is calculated from the clock, so it is correct when you return to the tab.
No. Official results need certified automatic timing.
Last checked: 29 October 2026.