What HIIT is and what 30/15 × 8 means
High-intensity interval training alternates short bursts of hard effort with short recoveries; the "high intensity" is the point, the intervals are just how you make it survivable. This page opens at 30 seconds of work, 15 seconds of rest and 8 rounds, the most common HIIT setting on timer apps: a 2:1 ratio, 5 minutes 45 seconds of rounds, plus a 10-second get-ready before the first one.
Thirty seconds is long enough for 10–15 repetitions of a bodyweight movement or a hard push on a bike, and fifteen seconds of rest is just enough to shake out and reset without recovering fully, which keeps the heart rate high across the whole block.
The presets: pick a ratio in one tap
HIIT 30/15 × 8 is the default. 40/20 × 8 keeps 2:1 with longer stations, good for circuits where you move between exercises. 45/15 × 10 is the boot-camp 3:1 for a longer burn. Tabata 20/10 × 8 is the 1996 protocol and has its own page. EMOM 60 × 10 removes rest: start a set every minute, rest what is left. Boxing 3:00/1:00 × 6 is six professional rounds.
A preset changes only work, rest and rounds; the get-ready, warm-up and cool-down you set stay. Type your own numbers in the boxes for anything else, from 1 to 3600 seconds and up to 99 rounds, and the total time updates before you start.
Building a session: warm-up, rounds, cool-down
Set a warm-up of 180–300 seconds when the intervals are your whole workout: easy movement that raises the pulse and rehearses the exercise you are about to do hard. Then the rounds. Then a cool-down of 120–180 seconds of walking or easy pedalling, which brings the heart rate down gradually rather than stopping dead. Both are 0 by default so the timer stays quick to start.
For a longer workout, run the timer twice with a few minutes between, or raise the rounds; most HIIT sessions are 10–20 minutes of rounds, and quality in the last rounds matters more than adding more. If the last two rounds are visibly slower, lengthen the rest or cut rounds next time.
Beginner to advanced: a progression
Beginner: 20/40 × 6–8 or 30/30 × 8–10, rest equal to or longer than work, two sessions a week. Intermediate: 30/30 × 8–12 or 40/20 × 8–10, three sessions. Advanced: 45/15 × 10–12 or this page's 30/15 × 8 done at a genuinely hard pace, and Tabata 20/10 × 8 for a short maximal block. These are the ranges that trainer guides broadly agree on.
Progress one variable at a time: first add rounds, then shorten rest, then lengthen work. Keep the same movement for a few weeks so you can tell whether the numbers are getting easier, and use the finish summary, which counts only rounds you completed without skipping.
Beeps, tone and the alarm
Three short beeps mark the last 3, 2, 1 seconds of every period and a higher tone marks the change, so you can keep your eyes on your form; the get-ready ends with the same tone, your GO. After the last round the alarm you chose repeats until you stop it. The sounds are made in the browser and pre-scheduled on its audio clock, so they land on the second even if the page is busy; the 3-2-1 beeps can be turned off, and None keeps only the colours and tab title.
On a phone: screen, background and iPhone
Keep screen on is on by default on phones, so the session is not cut by auto-lock, and F gives a full-screen view with the colour, seconds and round readable from the floor. Each period's end time is fixed when it starts, so the timer is exactly on schedule after any glance away or a reload. On an iPhone keep the tab in front, since Safari pauses hidden pages; iOS 17+ plays the beeps through the silent switch, and Android phones vibrate at the end.
HIIT, Tabata and the other timers
Tabata is a specific HIIT protocol, 20/10 × 8 at maximal effort, with its own fixed page; everything else is set here or on the interval timer, which is this page with the hub's defaults. The stopwatch times a run or a set with laps, and the plain timer counts one length. All share the same sounds and settings, and every setting here has a shareable address.