45s work / 15s rest × 10 rounds
HIIT Timer
High-intensity interval training is less a single workout than a principle: alternate hard efforts with recovery so you spend more total time at a high heart rate than a steady session would allow. This timer opens on a sensible general-purpose structure — 45 seconds of work, 15 seconds of rest, ten rounds — but the durations and the round count are all adjustable, because the right numbers depend on what you are training for. The lever most people ignore is the work-to-rest ratio. The default here is 3:1, which is firmly in conditioning territory: short recoveries keep your heart rate elevated and build tolerance to fatigue. Lengthen the rest toward a 1:1 or 1:2 ratio and you protect power output, which is what you want for sprint or heavy-effort intervals where each rep should be fast and clean rather than a grind. Same timer, very different training effect. That makes HIIT suit a wide range of people — someone doing kettlebell circuits, a runner doing hill repeats, a cyclist on a trainer — as long as they match the intervals to the goal instead of copying a number off a screen. Beginners are usually better served by more rest and fewer rounds; the intensity, not the volume, is what drives the adaptation. The non-obvious tip is about pacing effort, not just time. Aim for roughly an 8 to 9 out of 10 on the work intervals — hard enough that talking is difficult, but short of the absolute maximal sprint that defines true Tabata. If you red-line every interval at a 10, you will fade badly by round six and the back half of the session falls apart. Pick an effort you can repeat honestly across all ten rounds, and let the last two be the ones that hurt. A practical note on alerts: the work and rest sounds are reliable while this tab stays open, so prop your phone where you can hear it. Switch to another app and the browser may hold the next cue back by up to a minute; close the tab and nothing fires at all, since there is no server keeping time for you. On iPhone, the side mute switch silences web audio — flip it off first.
HIIT
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Frequently asked questions
- How is this different from Tabata?
- Tabata is a fixed, maximal-effort version of HIIT — 20/10 for eight rounds. This HIIT timer lets you set any work, rest, and round count at a sustainable hard effort rather than an all-out one. Use Tabata for four brutal minutes; use HIIT for a longer, adjustable session.
- What work-to-rest ratio should I use?
- For conditioning, shorter rest (2:1 or 3:1, like the 45/15 default) keeps intensity high. For power and speed work, use equal or longer rest (1:1 to 1:2) so each effort stays fast and clean. Match the ratio to the goal.
- How many rounds is enough?
- Ten rounds of 45/15 is about ten minutes of intervals, which is plenty for most people. If you can comfortably do far more, your work intervals probably are not hard enough. Start conservative and add rounds as you adapt.
- Do I need to warm up?
- Yes. Five to ten minutes of easy movement before the first hard interval lowers injury risk and actually improves your output — cold muscles cannot produce their best effort safely.
- Will it keep time if I lock my phone or change apps?
- The count is based on the real end time, so it stays accurate. But a backgrounded tab can delay the audio cue by up to a minute, and a locked or sleeping screen — especially on an iPhone — may not alert you at all; a fully closed tab will not either. Keep the tab open with the screen awake, and on iPhone keep the side mute switch off.
This timer keeps the exact time even if you switch tabs. If this tab or your browser closes, nothing can alert you at the exact finish time — there's no server behind this, by design. Come back any time and we'll tell you honestly how long ago it ended.