How Does a Stopwatch Work?

A stopwatch measures how much time passes between two moments. The principle is simple: count something that repeats at a steady, known rate. The difference between stopwatches is what they count.

Mechanical stopwatches

A mechanical stopwatch uses a wound spring and a balance wheel, which swings back and forth at a steady rate. Each swing lets a gear advance by one small step, and the gears drive the hands. Common models can show fifths or tenths of a second.

Digital stopwatches

A digital stopwatch counts the vibrations of a small quartz crystal. Most watches use a crystal that vibrates 32,768 times per second. A chip counts these vibrations and turns them into the digits you see. Because this rate is so stable, inexpensive digital stopwatches are very accurate.

Stopwatches in a browser

A stopwatch in a web page, like our online stopwatch, works a little differently. It doesn't count ticks at all. Instead it reads a high-resolution clock when you press Start, then subtracts that reading from the current one every time the display updates. Because the elapsed time always comes from clock readings, a busy page might update the display less often, but it never loses time.

This clock is "monotonic", which means it only ever moves forward. Your computer's ordinary clock can jump when it syncs with a time server or changes for daylight saving. The monotonic clock doesn't.

Lap times and splits

• Lap time: how long a single segment took.
• Split time: the total elapsed time at the end of that segment.

Runners use splits to check their overall pace, and laps to compare how consistent each stretch was.

How precise is it really?

Display precision and real accuracy aren't the same. Milliseconds on screen don't help if the person pressing the button reacts about 0.2 seconds late. That's why serious races use automatic sensors. For training and everyday use, the slight human error tends to even out, particularly when the same person presses start and stop.