Stopwatch

How accurate is an online stopwatch?

This one notes the system time when you press Start, and whenever it shows the elapsed time it subtracts that from the current time. Nothing accumulates, so nothing can drift: leave it running in a background tab for an hour, and when you come back the reading is exact, even though the browser was only updating the hidden page now and then. The clock is read in milliseconds and shown to hundredths, rounded down, so it never displays time that has not yet passed.

The weak link in any hand-operated stopwatch is the hand. Reaction time to a visual signal is typically around a fifth to a quarter of a second, so a manually timed run is uncertain by about that much at each end — which is why official athletics timing is automatic. People usually react a little faster to a sound than to a sight, so start on the signal you hear where you can. For comparing your own laps, consistency matters more than absolute accuracy: press the same way every time.

Laps, splits and the lap table

Pressing Lap records the moment and starts a new lap. The table shows two figures for each lap: the lap time, which is that lap alone, and the total, which is the time since the start — what athletics calls the split. Some sports and apps use 'split' for the lap time instead, so the columns here are labelled plainly. The lap in progress sits at the top of the table while it runs. Lap times are worked out from the same clock readings as the totals, so they always add up exactly to the total.

Once two or more laps differ, the fastest is marked in green and the slowest in red, with the words Fastest and Slowest so the marking does not rely on colour. Uneven pacing becomes obvious at a glance — a slow third lap in a 1,500-metre run, or a presenter who spends twice as long on one slide as on the others.

Keyboard shortcuts

Click the stopwatch or Tab to it, then: Space starts and stops, L records a lap and R resets once stopped. The shortcuts work only while the stopwatch has focus, so they never fire while you are typing elsewhere on the page and never clash with your browser's own shortcuts. If one of the stopwatch's buttons has focus, Space presses that button, as it normally would.

A key is often the more consistent way to time laps on a laptop, because there is no pointer to aim — useful beside a track, a pool or a lab bench.

Exporting laps to a spreadsheet

Download laps (CSV) saves a file with the lap number, the lap time and the total, then both times again as plain seconds — 12.345 rather than 00:12.34 — because spreadsheets interpret clock-style text unpredictably, while plain numbers can be averaged and charted straight away. If the stopwatch was stopped part-way through a lap, that final lap is included. Copy laps puts the same table on your clipboard, tab-separated, ready to paste into Excel, Google Sheets or Numbers.

What people time

Running and swimming laps, interval training and rest periods, speeches and debate rounds, Rubik's cube solves, science practicals, board-game turns, and how long a task really takes compared with how long it felt. Full screen turns the display into giant digits for a class or a team watching from across the room.

The stopwatch runs entirely in your browser; nothing is uploaded, and the laps exist only in this tab, so download them before you close it. The running time also shows in the tab title, so you can keep an eye on it from another tab.