Online Stopwatch

A free online stopwatch with lap times

This is a simple, accurate online stopwatch that counts up in hours, minutes, seconds and hundredths of a second. Press Start, press Lap to record split times, pause whenever you like and carry on from exactly where you stopped. It runs in your browser: there is nothing to install, no sign-up, and what you time stays on your device.

How to use the stopwatch

How this stopwatch measures time

A common way to build a web stopwatch is to add a fixed amount every time a timer fires, but browsers delay and batch timers, especially in background tabs, so that approach drifts. This stopwatch never counts ticks. When you press Start it records a timestamp from performance.now(), the browser's high-resolution monotonic clock. Every time the display updates, it simply subtracts that start timestamp from the current one and adds any time accumulated before your last pause. The screen is redrawn with requestAnimationFrame, so updates stay in step with your display whether it refreshes at 60, 120 or 144 Hz.

Because the elapsed time is recalculated from clock readings rather than accumulated, it stays correct even if the tab is hidden, the browser throttles it or the page stalls for a moment: switch away for a minute, come back, and the display catches up to the true value immediately. While it runs, the current time is also shown in the browser tab title, so you can glance at it from another tab; browsers throttle background tabs, so the title may refresh only about once a second or less. On devices that support it, the stopwatch asks the screen to stay awake while it is running; that request is released when the page is hidden and made again when you return.

What people use a stopwatch for

• Workouts and intervals: time sets, rest periods, planks and sprints, and use laps to log each round.
• Running and cycling laps: record every lap and see your fastest and slowest at a glance.
• Cooking: track how long something has been resting, proofing or simmering when a countdown isn't needed.
• Studying and focus sessions: practise Pomodoro-style work blocks, or see how long a past-paper question really takes you.
• Speaking practice: rehearse a talk, presentation or toast and note the time of each section with laps.
• Kids' games: hold-your-breath contests, races, tidy-up challenges and turn-taking.

A note on precision: the browser clock this reads typically has a resolution of about 0.1 millisecond or finer than the hundredths of a second shown, though browsers may coarsen it. As general knowledge rather than a measured figure, human reaction time when pressing the buttons is usually a couple of tenths of a second, so this is great for everyday timing but is not a certified sports timer and shouldn't be used for official results.

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