Stopwatch

About the Stopwatch

A precise browser-based stopwatch with lap recording, split times, and cumulative totals. It runs entirely in your browser with no installation needed, and keeps ticking accurately even if you switch to another tab.

Lap and split times

Press the Lap button while the stopwatch is running to record a split. Each lap shows both the individual lap time and the cumulative total elapsed time, making it easy to compare intervals side by side.

Copying your results

Use the Copy button to export all recorded laps as plain text — useful for pasting into a spreadsheet, notes app, or training log. The export includes both split and cumulative times for every lap.

Accuracy

The stopwatch uses the browser's performance.now() API, which provides sub-millisecond precision. It is significantly more accurate than Date.now() and is not affected by system clock adjustments.

Common uses

• Sports & fitness — timing runs, swim laps, or interval training
• Cooking — tracking multiple timers without a dedicated device
• Presentations — keeping talks within a time limit
• Testing & debugging — manually timing processes or user flows

Stopwatch precision and timing accuracy

Web-based stopwatches using JavaScript's Date.now() or performance.now() are accurate to within 1-5 milliseconds under normal conditions. performance.now() provides sub-millisecond precision and is not affected by system clock adjustments. For professional sports timing requiring better than 1ms accuracy, dedicated hardware timers are used.

• performance.now() — sub-millisecond precision, monotonic (not affected by clock changes), used in performance profiling
• Date.now() — millisecond precision, based on system clock, may jump if clock is adjusted
• Reaction time — human reaction adds 200-300ms after seeing the stop signal; use split times for intermediate measurements
• Professional timing — World Athletics uses Omega timing systems accurate to 1/1000th of a second for track events

Frequently Asked Questions