Interval Timer

An interval workout is a pattern rather than a single length of time: a period of effort, a period of rest, and the pair repeated a set number of times. So the timer asks for three things, the work length, the rest length and the number of rounds, and then runs the whole pattern on its own. Around the rounds you can add a short count to get into position, a warm-up, a cool-down, and several sets with a longer rest between them. The display shows which phase you are in, the time left in it and the round you are on, large enough to read from the floor or across a gym: it turns blue while you work and stays plain while you rest, and a strip under it shows the whole workout and how much of it is left.

A sound marks the start of every work period and every rest period, which is the point of an interval timer: you follow the workout by ear and keep your eyes on the exercise. Work and rest have different sounds, and three short beeps count you into every change. Pause stops everything where it is, for a drink or an interruption, and starting again carries on from the same second of the same round.

Tabata timing is twenty seconds of work and ten seconds of rest, repeated eight times, for four minutes in all. It is named after the Japanese researcher Izumi Tabata, whose 1990s study of short, very hard intervals used that pattern, and it is the most common preset people look for. Other high-intensity interval training uses longer blocks, often thirty to sixty seconds of work with a similar or shorter rest, and circuits use one round per station.

Boxing and other combat sports train in rounds too: a professional boxing round is three minutes with a minute of rest between rounds, and amateur and training rounds are often shorter. All of these are the same timer with different numbers, so set the lengths once and the pattern runs as long as you need it.

The timer works out each change from your device clock rather than from a count of ticks, so a phase ends when it should even if the page is briefly in the background. The sounds for the next few minutes are handed to the browser’s audio system in advance, which keeps them on time in a background tab, and the screen is kept on while a workout runs wherever the browser allows it. What a browser can still interrupt is the sound: a phone that locks its screen, or a browser saving power, may pause a page’s audio, so keep the page in front during a workout, or turn on the notification that tells you when it is over.

Nothing about the workout is sent anywhere. The lengths you set are remembered in your own browser for next time, workouts you save by name are kept there too, and there is no account and nothing to install. For a single length of time rather than a repeating pattern, the plain timer is simpler, and the stopwatch measures how long something took rather than counting down.