High Precision Stopwatch

Why a browser stopwatch needs to be built carefully

A naive stopwatch implementation just increments a counter every second using setInterval — but browser timers aren't perfectly reliable under load; background tabs get throttled, and accumulated small delays cause real drift over long sessions. A precise stopwatch instead tracks actual elapsed wall-clock time (using Date.now() or performance.now() as a reference point) and calculates the displayed time from that reference on each update, rather than trusting that every interval tick fired exactly on schedule. This is the difference between a stopwatch that's accurate for a 5-minute workout and one that silently drifts by whole seconds over a long recording session.

Lap timing, and why it's more than just "mark a point"

Lap functionality needs to record both the cumulative time since start and the time since the previous lap, since both numbers matter for different purposes — a runner tracking splits cares about lap-to-lap pace, while total elapsed time tells them their overall performance against a target. This tool records both automatically each time you hit lap, so you don't have to do the subtraction yourself afterward.

Common uses

• Workouts and interval training: Timing sets, rest periods, or full session length.
• Cooking: Tracking multiple simultaneous steps without a physical kitchen timer.
• Studying and productivity: Timing focused work sessions (e.g., Pomodoro-style intervals).
• Presentations and speeches: Practicing timing before a talk or exam with a spoken component.

Using this stopwatch

Press start to begin timing, lap to record a split without stopping the clock, and stop to end the session — all calculations happen locally in your browser tab, so keeping the tab open and active is important for continuous accuracy during long sessions.

Common questions

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Page last reviewed: July 2026