Guide

How accurate is an online stopwatch?

Key takeaways

The stopwatch measures elapsed time with your browser's steady clock, so the clock itself is rarely the weak point.
The biggest error usually comes from the person pressing the buttons, at both the start and the stop.
Sleep, a frozen tab or a changed system clock can disturb timing; the stopwatch warns you when it notices this.
Use it for practice, lessons and everyday tasks, not where certified timing is required.

How does the stopwatch measure time?

When you press Start, the page notes a reading from the browser's monotonic clock, a counter that only moves forward and is not affected when someone changes the computer's date or time. Every reading after that is compared with the starting one. The digits you see are redrawn each time the screen refreshes, but the stored time is calculated from the clock, not from counting screen frames. That is why a laggy display does not make the result drift.

Where does the error come from?

From most to least important in everyday use:

Your reactions. You press Start a moment after the event begins and Stop a moment after it ends. The two delays partly cancel out if they are similar, but they are rarely identical, and they differ from person to person and from attempt to attempt.
Anticipation. If you can see the finish coming, you may press early. A start signal you cannot predict usually makes you press late.
The input device. A touch screen, keyboard or mouse adds a small, varying delay before the browser receives the press.
The device itself. A computer that sleeps, or a tab the browser freezes to save power, can interrupt timing (see below).

So two people timing the same event can get results that differ by more than the hundredths shown on the screen.

What happens if my device sleeps or the clock changes?

The stopwatch keeps two references: the steady browser clock and the system clock. If they disagree after the computer has slept or someone changed the time, it shows a timing-uncertainty notice and keeps the larger elapsed value it can justify. Treat a session with that notice as approximate. To avoid it, turn on Keep screen awake (where your browser supports it) and leave the tab open in front.

How can I get more consistent results?

Let the same person press Start and Stop each time.
Use a clear, sharp signal, such as a hand drop or a word, rather than a gradual one.
Watch the event, not the screen, and use the keyboard (Space and L) on a computer.
Repeat the measurement and look at the average lap rather than a single attempt.

Is it good enough for official results?

No. Races, competitions and tests with formal rules use certified electronic timing or trained timekeepers following set procedures. An online stopwatch is right for training, classroom experiments, presentations and household tasks.

Questions

Why can I hide the hundredths? For calmer viewing on a projector or across a room. The measurement underneath does not change.

Is a phone app more accurate than this website? Both depend on the same factors: the device clock and, above all, the person pressing the button.