Rounds, rests and a countdown you can hear

Pick a preset or type your own numbers, check the total at the bottom of the display, then press Start. The timer runs warm-up, then every work and rest pair in order, then the cooldown. Sound carries further than a screen does, so the phase you are in has its own tone: a rising two-note cue into work, a lower single note into rest, and three beeps when the whole session ends. The last three seconds of every phase tick, which is your cue to get into position.

The countdown is anchored to the system clock rather than to a ticking counter. It reads the real time on every frame and works out which phase you should be in, so locking the phone or switching apps does not make the workout drift. Pausing stores how far you had got and rebuilds the schedule from the moment you resume. While a workout is running the page asks the browser to keep the screen awake, which most mobile browsers honour if the tab stays in the foreground.

The presets are the common formats rather than prescriptions. Tabata is 20 seconds hard and 10 seconds easy, usually run for 8 rounds. It comes from Izumi Tabata's 1996 study, which tested a routine devised by the Japanese national speed skating team's head coach on physical education students riding exercise bikes: seven or eight exhaustive sets with the work intervals at roughly 170 percent of VO2max. Anything gentler is a 20/10 interval workout, which is fine, just not what the study measured. EMOM means every minute on the minute: set rest to 0 so each round starts one minute after the last, and whatever time your reps leave over is your rest.

Interval work is demanding on the heart and joints. If you are new to it, coming back from injury, or have a heart condition, start with longer rests and fewer rounds, and get medical advice before doing maximal intervals. This page times the work, it does not judge whether the work suits you.

The card headed This workout is the whole session written out in order, and the number beside the title is how many separate phases that comes to. Take the Tabata preset: sixteen phases, which is one ten-second warm-up, eight work phases of twenty seconds and the seven ten-second rests that sit between them. There is no ninth rest, because a rest after the final round has nothing to lead into. Four minutes in total, of which two minutes forty is work.

That pair of figures is what the boxes under the clock show. Total is everything in the list added up. Working is your work length multiplied by rounds and then by sets, so the gap between the two, here one minute twenty, is the warm-up and the rests. Watch Total rather than the round count when you only have twenty minutes.

Where sets change the arithmetic

Ask for three sets of eight rounds at forty on, twenty off, with a thirty-second warm-up, ninety seconds between sets and a thirty-second cooldown, and the list comes to forty-nine phases running twenty-seven minutes, sixteen of them work. Only two set rests appear rather than three, on the same principle as the missing final rest. The list prints the first forty phases and then a line reading and 9 more.

The limits, and what happens when you pass them

Warm-up accepts 0 to 900 seconds, work 1 to 3600, rest 0 to 3600, rounds 1 to 99, sets 1 to 20, rest between sets 0 to 1800, and cooldown 0 to 1800. Type something outside one of those ranges and it is quietly clamped back inside rather than refused; a decimal is rounded, and a field you empty falls to its own floor. Work, rounds and sets all have a floor of one, so a zero typed into any of them comes back as one; setting warm-up, rest, cooldown or the rest between sets to zero drops that phase from the schedule, which is how a warm-up or a cooldown disappears. Rest between sets is only shown while sets is above one.