Stopwatch

What is a stopwatch?

A stopwatch is a timer that counts up from zero. You press Start, it begins counting, you press Stop when whatever you're measuring is finished. The word itself dates to the early 1700s, when watchmaker George Graham built the first hand-held mechanical version for British horse racing. The basic idea hasn't changed in 300 years: measure elapsed time, as accurately as the device allows, between two moments you choose.

What has changed is the precision. Graham's stopwatch could resolve a quarter of a second. The mechanical chronographs used at the 1932 Olympics resolved a tenth. Modern electronic stopwatches resolve a hundredth. The Stopwatch on this site uses the browser's high-resolution performance clock, which is accurate to roughly a millisecond — about as good as you can get without specialized hardware.

The other big difference between a stopwatch and a regular clock: a stopwatch doesn't care what time of day it is. It only cares about durations. That makes it the right tool whenever you want to know "how long did this take?" — a question that comes up more often than you'd think, in more places than you'd expect.

How to use the Stopwatch

The Stopwatch is a single page with three buttons and a running clock. There's no sign-up, no account, and nothing leaves your browser.

1. Click Start. The clock begins counting up from 00:00.00.
2. While it's running, click Lap at any point to record a split. The lap time is the elapsed time at that moment; the lap interval is the time since your last lap (or since the start, if this is the first lap).
3. Click Stop to pause. The clock holds at the current time.
4. Click Start again to resume from where you paused.
5. Click Reset to clear the time and lap history back to zero.

The display shows minutes, seconds, and hundredths of a second. The lap list appears below the main display and stays visible until you reset. The clock keeps running even if you switch tabs — it uses timestamps rather than tick counts, so background throttling doesn't slow it down.

How accurate is a browser stopwatch?

This is the question that comes up first whenever a stopwatch matters. The short answer: accurate enough for almost anything a person would measure with one. Browser stopwatches read from the same hardware clock the operating system uses, and modern hardware clocks drift by less than a millisecond per second on a properly calibrated machine.

This is why competitive sports use automatic timing systems with photocells and pressure plates, not manual stopwatches. The 100m sprint is decided by hundredths of a second — far below the noise floor of any human-operated device. For training, coaching, and everyday measurements, a millisecond-accurate stopwatch is overkill in a good way: the precision the clock offers is more than the precision your fingers can deliver.

There's one exception worth knowing about. If you leave the tab in the background for a long time, some browsers throttle JavaScript execution to save battery. The Stopwatch works around this by reading the system timestamp on every screen update, not by counting individual ticks — so even if the tab was throttled, the displayed time is correct when you return.

A worked example: timing a 5-lap run

Say you go to a 400-meter track for a 2 km run — that's exactly five laps. You want to know your total time, your per-lap splits, and your average lap. Here's what the stopwatch shows you, line by line, with realistic times for a recreational runner:

Final time: 9:25.74. To find your average lap, divide the total by the number of laps: 9:25.74 ÷ 5 = 1:53.15. The fastest lap (1:50.04) and the slowest lap (1:58.42) differ by 8 seconds — useful information, because it tells you where your pacing is uneven. A runner whose lap times vary by 8 seconds across a 5-lap run will benefit more from pacing drills than from raw speed work.

The Lap button is what turns a stopwatch into a coaching tool. Without splits, you just get a total. With splits, you can see exactly which laps slowed you down and decide what to fix.

Where stopwatches earn their keep

People reach for stopwatches in a surprising range of situations. A few of the most common:

• Running and cycling intervals. Track workouts (400m repeats, ladder sets) live and die by accurate splits. Most coaching plans assume you have lap-time data.
• Public speaking practice. Rehearsing a 5-minute pitch? Time it. The pitch you think is 5 minutes is usually 6:30, and the pitch you think is 6:30 is usually 9. Audio recordings lie about pace; the stopwatch doesn't.
• Recipe timing. Eggs are the classic case: a 6-minute soft-boiled egg is a different thing from a 7-minute soft-boiled egg. Kitchen oven timers usually round to the minute, which isn't precise enough.
• Music practice. Drum rudiments, scale exercises, anything you need to play for a fixed duration at a fixed tempo. Stopwatch + metronome is the practice room standard.
• Process improvement. "How long does our checkout flow really take?" Open a stopwatch, walk through the flow as a user, click Lap at each step. The number you get back is almost always longer than the number anyone on the team would have guessed.
• Board games. Chess clocks, Scrabble timers, anything with a per-move limit. Two stopwatches running in parallel work fine if you don't have a dedicated game timer.
• Tooth-brushing for kids. The dentist says two minutes. Two minutes feels like ten when you're seven. A visible stopwatch settles the argument.

Most word counters either bury the count under sign-up walls or open with a 14-day trial. The same is true of stopwatches — there's a long list of "premium" timer apps with ads, accounts, and notification permissions. This one is just a stopwatch. Open it, hit Start, do the thing.

Stopwatch-relevant world records

To put millisecond accuracy in perspective, here are a few records that were decided by the stopwatch — or, more accurately, by the automatic timing systems that have replaced manual stopwatches at the elite level. The numbers below are the official marks as of late 2025.

The 100m sprint is the canonical case for why we don't time these by hand anymore: Bolt's record beat the previous mark by 0.11 seconds, which is shorter than a human reaction time. A manual stopwatch literally could not have resolved the difference. For everyday training, of course, none of this matters — the runner who shaves 5 seconds off a 5K is making progress whether the clock is mechanical, electronic, or written in chalk on a track wall.

Stopwatch vs. timer vs. clock

These three get mixed up constantly, and the distinction matters because the right tool depends on the question you're asking.

• A clock tells you what time it is. "It's 3:47 PM."
• A countdown timer counts down from a duration you set, and rings when it hits zero. "Bake for 25 minutes." Try the Countdown Timer for this.
• A stopwatch counts up from zero. You don't tell it how long; you tell it when to stop. "How long did the meeting actually run?"

The simplest test: if you know the duration in advance, you want a timer. If you want to find out the duration, you want a stopwatch. Pomodoro work blocks are countdowns (you know they're 25 minutes); race times are stopwatches (you find out at the finish line).

Related tools

The stopwatch pairs naturally with a few other utilities on the site:

• Countdown Timer — the inverse of a stopwatch; counts down from a set duration and alerts you at zero.
• Pomodoro Timer — a specialized countdown that alternates work and break blocks for focused work sessions.
• Hours Calculator — adds up a list of durations or time spans. Useful for totaling stopwatch readings across a workout or workday.
• Date & Time Calculator — does arithmetic on calendar dates and clock times rather than raw durations.

Frequently asked questions

Will the stopwatch keep running if I switch to another tab?

Yes. The clock reads the system timestamp on each update, so even if the browser throttles the tab in the background, the time displayed when you return is accurate to the millisecond. Closing the tab entirely, however, resets the stopwatch — there's no autosave by design.

How many laps can I record?

There's no fixed limit. The lap list scrolls if it grows past the visible area. In practice, anything over a few dozen laps gets unwieldy on screen — at that point you'd want to copy the times into a spreadsheet for analysis. Hundreds of laps work fine technically.

Is the stopwatch accurate to one hundredth of a second?

The display shows hundredths, and the underlying clock is accurate to roughly a millisecond — ten times finer than what's shown. The real limit is human reaction time at the Start and Stop button presses, which is around 250 ms on average. So while the clock is precise, a manual reading carries ±0.2 s of human error. For sports or scientific timing where this matters, automatic triggers (photocells, sensors) are required.

Why does the time display look slightly different in different browsers?

Browsers use slightly different fonts and rendering for tabular numerals, so the column widths of the digits vary by a few pixels. The numeric value is identical across browsers — it's just visual.

Can I export my lap times?

Not as a built-in feature. The simplest method is to highlight the lap list with your cursor and copy-paste into a spreadsheet or text file. Adding a CSV export is on the roadmap; if you want it, the request goes furthest if you describe the workflow you'd use it for.

Why does my stopwatch sometimes seem off by a fraction of a second between Start and the displayed zero?

The display refreshes about 60 times per second to keep the digits readable. The clock itself starts counting at the precise moment you click Start, but the first visible frame of the count may not appear for up to 16 milliseconds (one frame at 60 Hz). The recorded times use the actual click timestamps, so the underlying numbers are correct even if the visual frame lags by one tick.

Can I use the stopwatch for scientific experiments?

For experiments where ±100 ms accuracy is acceptable — most physics lab measurements at the high-school and introductory undergraduate level — yes. For anything requiring tighter precision (reaction-time studies, sub-second physiology, motion capture), you want hardware sensors with electronic triggers rather than a button-pressed stopwatch. The limiting factor isn't the clock; it's the human between the experiment and the click.

What happens to my data if I refresh the page mid-run?

It resets. The stopwatch state lives in the browser tab's memory, not in storage. This is deliberate — a stopwatch is a real-time instrument, not a record-keeper. If you need a permanent log, write the time down or copy it to another file before refreshing.