Stopwatch & Timer — Lap Times, Countdown & Alarm

Online Stopwatch & Timer

This page combines a count-up stopwatch and a countdown timer in one view. The stopwatch shows hours, minutes, seconds and hundredths of a second and records lap times while it runs; the timer counts down from any combination of hours, minutes and seconds, drains a progress ring as it goes, and finishes with a Time's Up notice and an alarm sound.

How the stopwatch measures time

The stopwatch stores a system-clock timestamp at the moment it starts and calculates the display as the difference between the current clock reading and that timestamp. Pausing adds the finished span to a stored total, and resuming opens a new span, so nothing is lost between pause and resume, not even a hundredth. Because elapsed time comes from the clock rather than from a frame counter, switching to another browser tab does not slow the count. While the stopwatch or the timer runs in the background, the current reading is also written into the tab title, so it stays visible from any other tab.

Reading lap times

A lap records the total elapsed time at the instant it is taken and places the newest entry at the top of the list, formatted as minutes:seconds.hundredths. The duration of a single lap is the difference from the previous entry. A swimmer timing 50 m lengths might record 00:42.15, 01:26.90 and 02:13.40. The first length took 42.15 seconds, the second 1:26.90 − 0:42.15 = 44.75 seconds, and the third 2:13.40 − 1:26.90 = 46.50 seconds. Resetting the stopwatch clears the lap list along with the elapsed time.

What the countdown timer does

The timer converts the hours, minutes and seconds you set into one duration and fixes an end moment on the system clock; the remaining time is recalculated against that end point, so it stays correct even after the tab has been inactive. The display counts in whole seconds, rounded up, and the ring around it empties in proportion to the time left — at half the duration the ring is half drawn. When the countdown reaches zero, a Time's Up notice appears and an alarm sound plays. Pausing freezes the remaining time; resetting stops the alarm and returns to the editable duration.

Common countdown durations

Durations often set on this timer, from the kitchen to the desk: