Online Stopwatch

What is the Online Stopwatch?

This online stopwatch is a free, browser-based timer that counts up from zero with millisecond precision. Press start and it measures exactly how long something takes — a sprint interval, a chemistry reaction, a timed practice question — and it can record lap splits along the way without ever stopping the clock. There is nothing to install and no account to create: open the page and you are already timing.

How to use it

1. Press the Start button (or tap the Spacebar) to begin counting from 00:00.000.
2. While it runs, press Lap (or the L key) each time you want to mark a segment. Each lap is saved with its own split time and the running total.
3. Press Stop (or Spacebar again) to pause. The reading freezes exactly where it is; press start again to resume from that point.
4. Press Reset (or R) to clear the time and the lap list back to zero.

The whole thing lives on one screen, so you get your answer at a glance and move on.

The method behind it: why performance.now() instead of Date

A naive stopwatch counts time by adding up setInterval ticks — "another 10 milliseconds passed, another 10..." That approach drifts, because timers are not guaranteed to fire on schedule and the browser deliberately slows them down in background tabs. The error accumulates the longer you run.

This stopwatch avoids that entirely. It records a single start timestamp from performance.now() — a high-resolution clock that always moves forward and never jumps when the system clock changes — and on every frame it simply computes elapsed = now − start. Because the elapsed time is always derived from two fixed points rather than summed tick by tick, it cannot drift. Lap splits use the same math: each split is just the difference between two recorded totals, so the cumulative column always adds up exactly.

Examples

• Interval run: Start at the whistle, hit Lap at the end of each 400m. Lap 1 reads 1:32.41 split / 1:32.41 total; Lap 2 reads 1:30.88 split / 3:03.29 total — letting you compare each lap instantly.
• Practice exam: Start the clock, work a question, hit Lap when you finish it. You end with a per-question split list showing exactly where you spent your time.
• Cooking a quick sear: Start when the steak hits the pan, Stop at the flip — a clean 1:45.20 with no app and no distractions.

Common use cases

• Athletes and coaches timing sprints, intervals, and rest periods.
• Students timing themselves on practice tests and individual questions.
• Scientists and lab workers timing reactions and procedures.
• Speakers and presenters rehearsing to fit a time limit.
• Anyone who just needs a fast, no-friction stopwatch in a browser tab.

Why use this one

Most browser stopwatches stop at start/stop/reset. This one adds lap splits so you can break a session into segments without losing the running total, keyboard control (Spacebar to start/stop, L to lap, R to reset) so you never fumble for a button at the critical moment, and drift-free precision from performance.now() so long sessions stay accurate to the millisecond. It is deliberately minimal — the time is the whole page — runs 100% in your browser, requires no signup, and keeps every timing on your own device.

Frequently asked questions