Online Stopwatch
Millisecond-precision stopwatch with lap times. Works entirely in your browser.
Lap Times
Frequently Asked Questions
How accurate is this stopwatch?
The display updates every 10 milliseconds using JavaScript's Date.now(). Accuracy is typically within 10–20ms depending on your browser and system load.
Can I record lap times?
Yes - Click the "Lap" button while the stopwatch is running to record a split time. Each lap shows the lap time (delta) and the cumulative total. The fastest lap is highlighted in green, slowest in red.
Does it work in the background?
The stopwatch continues running if you switch tabs or minimize your browser, as it uses Date.now() rather than incremental counting.
Stopwatch World Records
Some of the most awe-inspiring moments in human achievement are measured in fractions of a second. These records put millisecond precision in context.
The fastest human ever recorded over 100 metres. Bolt's average speed across the race was 37.58 km/h (23.35 mph), with his peak speed hitting approximately 44.72 km/h between the 60 and 80 metre marks.
Red Bull's pit crew changed all four tyres on Max Verstappen's car in 1.80 seconds at the 2023 Qatar Grand Prix. The choreography requires over 20 mechanics operating in precise synchrony, each responsible for a single task lasting under a second.
Australian Sam Groth served at 263.4 km/h (163.7 mph), the fastest officially recorded serve in professional tennis. At that speed, the ball travels the 23.77 metres of a service box in under 0.33 seconds - Barely enough time for a human to blink.
On May 6, 1954, Roger Bannister became the first person to run a mile in under four minutes, clocking 3 minutes 59.4 seconds. Bannister used a scientific approach to pacing and trained specifically around the physiological demands of the milestone. Forty-six days later, John Landy broke it again.
19th-century surgeon Robert Liston was renowned for speed - Before anaesthesia made surgery longer and safer, fast operating reduced patient suffering and shock. He is recorded to have amputated a leg in under 35 seconds. Modern surgery, by contrast, optimises for precision over speed.
How Millisecond Precision Works
Measuring short durations accurately in a web browser is more nuanced than it might appear. The most obvious approach - Calling Date.now() every millisecond - Falls short because Date.now() returns the number of milliseconds since the Unix epoch and is subject to system clock corrections (including NTP adjustments and leap second corrections), which can cause sudden jumps. For a stopwatch, a jump in the underlying clock translates directly to a visible glitch in the displayed time.
The browser's performance.now() API solves this by returning a high-resolution timestamp that is monotonically increasing - It never goes backwards and is not affected by system clock changes. It is measured in milliseconds with sub-millisecond fractional precision (typically microsecond resolution on modern hardware). This stopwatch uses Date.now() for elapsed time tracking, which provides sufficient precision for centisecond display and correctly handles background tab behaviour.
The display updates on a 10ms interval using setInterval. Browsers throttle setInterval to roughly 1000ms when a tab is in the background, which means the display will not update in real time when you switch away - But because the elapsed calculation uses actual wall-clock time at each update, the value shown when you return to the tab will be accurate.
Use Cases for an Online Stopwatch
- Sports Timing Time running intervals, swim laps, cycling sprints, or any athletic drill. The lap function records splits so you can review performance by segment.
- Cooking Time boiling eggs, resting meat after cooking, or marinating ingredients. The lap button lets you track multiple steps in a recipe simultaneously.
- Presentations Practice a pitch or speech with the stopwatch running to calibrate your pacing. Lap at the end of each section to measure how long each part takes in rehearsal versus live.
- HIIT Workouts Track work and rest intervals for high-intensity interval training. While a dedicated interval timer offers more automation, the stopwatch is useful for ad-hoc sessions or when you want full control over timing.
- Coding Sprints (Pomodoro) Start the stopwatch when you begin a focus session and check it periodically to stay within your intended time box. Lap at each natural breakpoint to build a record of actual time spent per task.
- Exam Practice Simulate timed exam conditions by starting the stopwatch at the beginning of a practice paper. Lap at each question or section to see where time is being spent and adjust strategy before the real test.
More Questions
How accurate is the online stopwatch?
The stopwatch is accurate to within 10–20 milliseconds under normal conditions. Accuracy can be affected by system load - If your computer is running many processes simultaneously, the JavaScript timer may fire slightly late. For scientific or official timing, purpose-built hardware stopwatches with dedicated clock circuits are required.
Does the stopwatch work in background tabs?
Yes - The elapsed time remains accurate when you switch tabs, because the stopwatch tracks wall-clock time (using Date.now()) rather than counting individual timer ticks. The visual display will not update in the background, but when you return to the tab the shown time will reflect the actual elapsed duration.
How do I record lap times?
Click the "Lap" button while the stopwatch is running. A new row appears in the Lap Times table below the display showing the lap number, the time for that specific lap (delta), and the cumulative total time. The fastest lap is highlighted green and the slowest in red once you have recorded two or more laps.
Can I time multiple events at once?
The stopwatch supports one running timer at a time. For multiple simultaneous timings, open the page in separate browser tabs - Each tab maintains its own independent stopwatch state. Alternatively, use the Lap feature to record splits within a single continuous run.