Online Stopwatch

How the elapsed time is measured

The stopwatch does not count its own ticks. When you press Start it records the clock time of that click, and on every redraw it subtracts that moment from the current clock time. The screen is refreshed roughly every 31 milliseconds, so the hundredths column advances in jumps of about three, but the value behind it is always the true elapsed time to the millisecond.

Stop adds the elapsed interval to a stored total, and Resume opens a new interval on top of it. You can stop and resume as often as you like without the pauses being counted. Hours only appear in front of the minutes once the total passes 59:59.99, so a short timing shows as 04:12.37 rather than 00:04:12.37.

Reading the lap table

Each row records the total elapsed time at the instant you pressed Lap. The Split column is the difference between that row and the row above it, which is what you want for per-lap pace or per-set rest. A worked example with three laps:

• Lap is only enabled while the stopwatch is running, so you cannot add a row after pressing Stop.
• Rows are appended in order and never edited or removed individually; Reset clears the whole table.
• There is no export button. To keep the laps, select the table with the mouse and copy it into a spreadsheet or note before resetting.

What happens if you switch tabs

Browsers slow down JavaScript timers in tabs you are not looking at, typically to one update per second and, after several minutes, to one per minute. The display therefore stops refreshing smoothly while hidden, but because the time is derived from the clock rather than from counted ticks, it is correct the moment you return. Only the on-screen redraw lags; the measurement does not.

Nothing is written to storage. Reloading the page, closing the tab or navigating away discards the time and every lap, so note the final result before leaving.

Stopwatch, timer or countdown: which one you need

Precision in practice

• The reading shown is accurate to within one redraw interval, about 0.03 seconds; the stored value is exact to the millisecond.
• Your own reaction time when pressing Start and Stop is usually 0.15 to 0.25 seconds each way, which is far larger than any error in the stopwatch itself. For sports results that need certified timing, use a dedicated device.
• If the device clock is adjusted while the stopwatch runs, for example by an automatic time sync, the elapsed value jumps by the same amount.
• There are no keyboard shortcuts; all control is through the three buttons, which also work by touch.