Stopwatch
A stopwatch accurate to the hundredth of a second with laps, best and worst lap, the change from the average, and pace and speed when you enter the lap distance. Keyboard controls, and it keeps running through a reload.
How to Use
- Press Start or the space bar. Press Lap or L at the end of each lap or split.
- Read each lap’s time, the running total, and how it compares with the average; the best lap is marked green and the slowest red.
- Enter a lap distance, such as 400 m for a running track, to see the pace and speed of every lap and of the whole run.
- Press Stop to pause, Start to carry on, and Reset (or R) to clear. Copy laps copies a table you can paste into a spreadsheet.
Worked Example
Four laps of a 400 m track: 1:32.4, 1:35.1, 1:34.0 and 1:30.5. The total is 92.4 + 95.1 + 94.0 + 90.5 = 372.0 seconds, or 6:12.0 for 1,600 m, and the average lap is 372.0 ÷ 4 = 1:33.0. The last lap was the best, 2.5 seconds faster than the average; the second was the slowest.
Pace per kilometre: 372.0 s ÷ 1.6 km = 232.5 s = 3:52.5 per km. Speed: 1.6 km ÷ (372.0 ÷ 3,600 h) = 15.48 km/h, or 9.62 mph.
The common mistake: averaging speeds instead of times. Lap speeds of 15.58, 15.14, 15.32 and 15.91 km/h average 15.49 km/h, close to but not the true 15.48 km/h, because slower laps last longer and should count for more. The right average speed is total distance over total time, which is how this stopwatch works it out.
Show Work
Formulas
Timing to the Hundredth
The first watches that could be started and stopped for timing races appeared in the early 19th century; Louis Moinet built a “compteur de tierces” in 1816 that measured to a sixtieth of a second. Hand-held stopwatches timed the Olympics until electronic timing became official, and hand times are still treated with caution: because a timekeeper reacts to the flash or smoke of the starting gun, statisticians conventionally add 0.24 seconds to a hand-timed sprint to compare it with an electronic one.
Today the best electronic systems start from the gun itself and read the finish with photo-finish cameras at thousands of frames a second, while swimming pads stop the clock at the touch. A browser stopwatch is limited mainly by the moment your finger presses the key.
About This Tool
This stopwatch measures to the hundredth of a second from your device clock, records any number of laps with their split times, highlights the best and slowest, shows each lap against the average, and turns a lap distance into pace and speed per kilometre or mile. Laps can be copied as a table, and the stopwatch carries on if you reload the page.
Everything runs in your browser; nothing is sent anywhere.
Related tools: Countdown Timer, Pomodoro Timer, and Speed, Distance & Time Calculator.
Frequently Asked Questions
How accurate is this stopwatch?
It reads your device clock, which has millisecond resolution, and shows hundredths of a second. The limit is your reaction time when pressing the button, typically 0.15 to 0.25 seconds, which is why races use electronic timing. Because the time comes from the clock rather than a count of screen updates, it does not drift, even in a background tab.
What is the difference between a lap and a split?
A lap time is the time for one lap on its own; a split is the total time from the start to the end of that lap. On a four-lap run, laps might be 1:32.4, 1:35.1, 1:34.0 and 1:30.5, and the splits 1:32.4, 3:07.5, 4:41.5 and 6:12.0. This stopwatch shows both.
How are pace and speed worked out?
Pace is time per unit of distance and speed is distance per unit of time. A 400 m lap in 1:33.0 is a pace of 93.0 ÷ 0.4 = 232.5 seconds, or 3:52.5, per kilometre, and a speed of 0.4 ÷ (93.0 ÷ 3600) = 15.48 km/h.
Does it keep running if I close the page?
Its start time and laps are saved in this browser, so if you reload or come back to the page it shows the right elapsed time and carries on. Nothing is sent anywhere.
Can I use the keyboard?
Yes: space starts and stops, L records a lap, and R resets. The keys work whenever you are not typing in a box.
How do I use the Stopwatch?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Running and swimming
Track laps on a 400 m track or a 50 m pool and see the pace of each one.
Interval training
Record each work interval as a lap and compare it with the average to see when you start to slow down.
Speeches and presentations
Time each section of a talk as a lap to find where you run long.
Lab work and cooking
Time reactions, steeping or proving with laps for each stage.
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