How to use the stopwatch
- Start begins timing; the same button then says Stop. Press Start again to carry on from where you stopped.
- Lap records the time so far. The table shows each lap’s own time and the total, newest at the top, with the fastest and slowest marked once you have three laps.
- Reset clears the time and the laps.
- On a keyboard: Space starts and stops, L records a lap and R resets.
How to read a stopwatch
The display reads minutes:seconds.hundredths, and hours come first once you pass an hour (1:02:05.37). Like a stopwatch in your hand, it never rounds up: 59.999 seconds shows 0:59.99, because a full minute hasn’t passed yet. Lap times are worked out from those same shown totals, so the laps in the table always add up exactly to the total.
Source NIST · Stopwatch and Timer Calibrations (SP 960-12) ↗
How this stopwatch keeps time
Browsers don’t promise to run a page’s timers on schedule: they slow them down when the computer is busy and especially when the tab is in the background. A stopwatch that counted its own ticks would fall behind. This one notes the moment you press Start and works the time out from your device’s clock each time it draws the display, so switching tabs, locking your phone or letting your laptop sleep can’t make it lose time. The display just catches up when you look again.
Source WHATWG · HTML Standard, Timers ↗
How accurate is a stopwatch?
The clock in your phone or computer is far more precise than anyone’s thumb. When you start and stop by hand, reaction time (tens to hundreds of milliseconds, according to NIST) is the biggest source of error. It cancels out partly, because you are late both when starting and when stopping, but for short times such as a 100 m sprint it can still decide who won.
FAQs
How to read a stopwatch?
Read it from the left: minutes, then seconds, then hundredths of a second. 2:05.37 is 2 minutes, 5 seconds and 37 hundredths. From an hour, hours come first: 1:02:05.37. Two digits after the point means the stopwatch shows time to the hundredth of a second (a resolution of 0.01 s).
Source NIST · Stopwatch and Timer Calibrations (SP 960-12) ↗
What is a stopwatch?
A stopwatch measures a time interval: the time between two events, starting from zero when you press Start. A clock shows the time of day instead.
What is the uncertainty of an iPhone stopwatch?
For any stopwatch you start and stop by hand, a phone’s included, your reaction time is the biggest uncertainty: NIST puts it at typically tens or even hundreds of milliseconds, far more than the 0.01 s the display shows. It matters most for short times.