How to use an interval timer

Work and rest in sequence, what each format actually prescribes, and how to tell whether a browser timer is keeping honest time.

What an interval timer is for

A plain countdown answers one question: how long until this ends. An interval timer runs a sequence — work, rest, work, rest — and tells you which part you are in without you having to look at a list and remember where you got to. That is the whole difference, and it is why a single timer cannot stand in for one.

The sequences below are the common ones. They are all the same three numbers underneath: how long you work, how long you rest, and how many times round.

The formats, and what they actually mean

Tabata

Twenty seconds of hard effort, ten seconds of rest, eight times. Four minutes in total, including the rests. It comes from a 1996 study by Izumi Tabata and colleagues on speed skaters, and the protocol as tested was genuinely maximal — not "brisk". Most of what is sold as Tabata is a twenty-on ten-off circuit at a sensible intensity, which is a fine way to train but is not the thing from the study.

HIIT

High-intensity interval training is a category, not a fixed pattern. Common shapes are 40/20, 30/30 and 45/15, run for eight to twelve rounds. The longer the work interval, the lower the intensity it can be held at.

EMOM

Every minute on the minute. A round starts each minute; you do the prescribed work and whatever is left of the minute is your rest. Finish faster and you rest longer, which makes the rest self-regulating. Set the work interval to sixty seconds and the rest to zero.

Circuits

Several stations, a fixed time at each, a short changeover between them, then the whole lap again. Use rounds for the number of stations and cycles for the number of laps, with the rest between cycles as your breather.

Pomodoro

Twenty-five minutes of work, five minutes off, four times, then a longer break of fifteen to thirty minutes. It is a working method rather than a training one, but the shape is identical, so the same timer runs it.

Setting one up

1. Get ready — the gap between pressing start and the first work interval. Ten to fifteen seconds is enough to put the phone down and get into position.
2. Work and rest — the two numbers that define the interval.
3. Rounds — how many work intervals in one lap.
4. Cycles — how many laps, with an optional longer rest between them.
5. Cool down — a final block, if you want the timer to hold you there.

The rest after the very last round is left out by default, because the session is over and standing still watching a clock run down serves no purpose. If you are running laps back to back and want it, there is a switch for it.

Why browser timers drift, and what to check

This is worth understanding, because it affects every timer you run in a browser tab, not just this one.

The straightforward way to build a countdown is to ask the browser to call you back every second and subtract one each time. It works perfectly while you are looking at it. The moment the tab goes into the background, browsers throttle those callbacks to roughly one a minute to save battery — so the subtraction happens far less often than once a second, and the timer falls behind. A ten-minute timer can finish minutes late, and nothing on screen indicates that it happened. People usually conclude they mis-set it.

The fix is not to count at all. Record the moment the session started, and whenever you need to draw the display, work out where you are from the current time. A tab that was asleep for twenty minutes simply computes the right answer when it wakes, because the answer never depended on being awake. That is how this timer works, and it is why the reading is correct the instant you switch back to it.

Sound is a separate problem with a separate answer. A beep fired from a throttled callback arrives late for the same reason. So instead the cues for the whole session are handed to the browser's audio clock in advance, at the moment you press start. The audio clock is not throttled, so those beeps sound at the right time whether or not the page is awake to hear about it.

How to check any timer for this: set it for two minutes, start it, switch to another tab for the full two minutes, then come back. An honest timer shows zero or an overrun. A drifting one still has thirty or forty seconds on the clock.

Getting the sound and screen right on a phone

Browsers will not play sound until you have interacted with the page, so the first press of Start is what enables it — there is no way around that and it applies to every site. Your phone's silent switch overrides the page as well.

Screens lock themselves after thirty seconds or so, which is exactly the wrong moment. The timer asks the browser to hold the screen awake while a session runs, using the wake lock feature. Some browsers refuse when the battery is low, and the lock is dropped whenever you leave the page and retaken when you return.

Frequently asked questions

Is Tabata really only four minutes?

The protocol itself is: eight rounds of twenty seconds on and ten seconds off comes to four minutes exactly. That does not include a warm-up, and the original study's intensity was maximal rather than merely hard.

What is the difference between rounds and cycles?

A round is one work interval. A cycle is one pass through all the rounds. Five stations done three times over is five rounds and three cycles, with the rest between cycles as your breather between laps.

How do I set up EMOM?

Set work to sixty seconds and rest to zero, then choose the number of rounds. Each round begins on the minute, and whatever time you have left after the work is your rest.

Will the timer keep time if I lock my phone or switch apps?

The countdown stays correct, because it is worked out from the clock rather than counted. Whether the alarm sounds depends on the browser keeping the tab alive; if it does, the beeps are already scheduled on the audio clock and will sound on time.

Why does my other online timer finish late?

Almost certainly because it counts interval callbacks, which browsers throttle to about one a minute in background tabs. You can test any timer for this: run it for two minutes with the tab in the background and see whether it comes back at zero.

Can I use it for something other than exercise?

Yes. Stretching and physiotherapy sets, practising an instrument, timed revision, presentation rehearsal, and Pomodoro work sessions all fit the same shape. There is a Pomodoro preset.