Four things decide the last digit
- The device clock. Stopwatch.ca uses the browser's high-resolution monotonic clock, which isn't affected by the wall clock changing. Browsers deliberately coarsen its resolution for security — to a fraction of a millisecond, or about a millisecond, depending on the browser — which is still finer than the thousandths digit.
- The screen. A 60 Hz display redraws about every 16.7 milliseconds, so while the watch runs the thousandths digit is a blur. The stopped time isn't affected: it's taken from the clock at the moment of your press, not from the screen.
- Input delay. Keyboards, mice and touchscreens take a few milliseconds to report a press. It's similar at start and stop, so it largely cancels out for an elapsed time.
- You. Simple visual reaction time is around a fifth to a quarter of a second, and it varies from press to press by a few hundredths or more. That variation is the real limit on hand timing.
Which precision to use
| Precision | Use it for |
|---|---|
| 1 second | Cooking, study sessions, meetings, anything long. |
| 1/10 s | Hand timing of races and events — matches the real accuracy of a human start and stop. |
| 1/100 s | Laps, training, most sport and lab work; what most hardware stopwatches show. |
| 1/1,000 s | Comparing very close results, keystroke-to-keystroke timing. Read the last digit as indicative. |
| 1/10,000 s | Watching the clock itself, comparing keyboards, mice and touchscreens. |
| 1/100,000 s | The finest step a browser timer resolves — about 5 µs in Chrome and Edge on this site. |
What 1/10,000 and 1/100,000 of a second really mean
At those digits you're reading the clock, not the event. Browsers deliberately blur their timers to defend against attacks that measure tiny time differences; a page that opts into cross-origin isolation, as Stopwatch.ca does, is given the finer timer — about 5 microseconds in Chrome and Edge. Browsers that don't offer it keep a coarser tick, and the digits finer than that tick are padding.
Open Timing details under the stopwatch to see your browser's measured tick, whether the page is isolated, and your screen's refresh rate. Even a perfect clock can't remove the variation in a human press, which is thousands of times larger than 10 µs — so these digits are for comparing devices, studying timing, or simply seeing how fine the measurement goes.
Hand timing versus automatic timing
Sport draws the same line. Track rules round hand-timed results up to the next tenth of a second and fully automatic results up to the next hundredth, and swimming's touchpads report hundredths. Hand times tend to read slightly faster than automatic ones, because a timer anticipates the finish but reacts to the start.
Stopwatch.ca truncates the display rather than rounding it: 12.349 shows as 12.34 at hundredths. If you need to report a result under a sport's rules, apply that sport's rounding.
Making your timing more consistent
- Use the same input for start and stop — the space bar for both, or a tap for both.
- Watch the event, not the screen.
- For a group, use the race mode so every runner shares one start.
- Use a countdown to start when you're timing yourself.