Setting up a workout
Pick a preset or set your own: how many seconds of work, how many of rest, how many rounds, and an optional warm-up before the first round (0 for none). A round is one stretch of work and the rest after it; the last round ends on work, with no rest after it, so Tabata (20 seconds on, 10 off, 8 rounds) comes to 3 minutes 50 seconds. The + and − buttons move in steps of five seconds; type any number for something else.
Watching, or just listening
The panel is filled purple during work and turns pale during rest, so you can tell the phase at a glance across a room. Three short beeps count down the last three seconds of each phase, a higher tone marks the change, and a chime marks the end. Under the time are the round you are on and the time left in the whole workout.
A link that keeps your workout
The address holds the workout as work, rest and rounds, then the warm-up if there is one: /interval-timer#40-20-6-30 is 40 seconds on, 20 off, 6 rounds after a 30 second warm-up. Bookmark it, or send it to someone you train with.
In a background tab, or with the screen off
The timer keeps the moment the workout started and works out the phase from your device's clock, so it stays right in a background tab, and the beeps come from a small background worker that browsers do not slow down. While it runs it asks the browser to keep the screen on, where the browser allows it. If the device sleeps anyway, the timer is back in the right place when it wakes, without a burst of the beeps it missed.
Questions
What is Tabata?
A form of high-intensity interval training from a 1996 study by Izumi Tabata: 20 seconds of all-out effort and 10 seconds of rest, eight times over. Many workouts now use the name for any 20/10 timing.
Can I skip a phase?
Yes. Skip jumps to the start of the next phase, and the time left in all is worked out again.
Why are there no beeps?
Check that Sound is on and the device is not muted. Browsers only let a page make sound after you have pressed something on it, which pressing Start does.