HIIT Workouts Explained — Benefits, Risks and Sample Sessions
High-intensity interval training — HIIT — is the most time-efficient form of cardio there is: short bursts of hard effort, separated by recovery, that improve fitness as much as much longer steady exercise in a fraction of the time. A 20-minute HIIT session can improve VO2 max as much as a 45-minute steady run, and it burns more calories per minute than any other form of cardio. It is also the most commonly done badly: intervals that are not intense enough to count, rest periods that are too short, and sessions done so often that recovery suffers.
The headline answer is that HIIT means repeated bouts of 20–60 seconds at 80–95% of maximum heart rate, separated by equal or longer recovery, for a total of 15–25 minutes, two to three times a week. The work must be genuinely hard — a perceived effort of 8–9 out of 10 — or it is not HIIT, it is just interval training at a moderate pace. The rest must be long enough to repeat the effort, or the intensity falls and the session becomes steady cardio in disguise.
Use the calculator
Start with your own numbers: try our heart rate zone calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Cardio vs Weights for Fat Loss, How to Warm Up Properly, Rest Day Guide. You can also browse the full guides section for the rest of the series.
The key points
HIIT improves fitness fast. Studies consistently find that HIIT improves VO2 max (the best measure of cardiovascular fitness) by 10–15% in 6–8 weeks, which is similar to or better than steady-state training in half the time. The mechanism is the high intensity: the heart, lungs and muscles are stressed more per minute, so the adaptation is greater. For people with little time, HIIT is the most efficient way to improve fitness that exists.
HIIT burns more calories per minute, not more per week. A HIIT session burns about 12–15 kcal per minute during the work intervals, against 8–10 kcal per minute for steady running — but the session is shorter, so the total is similar. The 'afterburn' (EPOC, excess post-exercise oxygen consumption) is real but small: about 6–15% of the session's calories, or 20–40 kcal, not the hundreds that marketing claims. HIIT is efficient, not magical.
HIIT is demanding and needs recovery. A true HIIT session is a hard training stress, and it needs 48 hours before the next hard session — the same as heavy lifting. Doing HIIT every day is a recipe for under-recovery, injury and burnout, especially for beginners. Two to three sessions a week, on non-consecutive days, with easier days in between, is the right dose for most people.
HIIT is not for everyone, at first. People with heart conditions, high blood pressure, joint problems or very low fitness should build a base of steady cardio first, and discuss HIIT with a clinician. Beginners should start with longer intervals at a lower intensity (30 seconds on, 90 seconds off, at 70–80% of maximum heart rate) and progress over weeks. The intensity that makes HIIT effective is also what makes it risky for the unprepared.
At a glance
| Protocol | Work : rest | Total time | Best for |
|---|---|---|---|
| Beginner | 30 s on : 90 s off | 15 min | Building a base |
| Standard | 30 s on : 30 s off | 20 min | General fitness |
| Tabata | 20 s on : 10 s off × 8 | 4 min | Short, very intense |
| Long intervals | 4 min on : 3 min off × 4 | 28 min | Endurance, VO2 max |
| Sprint intervals | 30 s sprint : 4 min walk × 6 | 27 min | Speed, power |
The formula behind it
The intensity prescription is 80–95% of maximum heart rate during work intervals, with maximum heart rate estimated as 220 − age (a rough estimate, not a measurement). For a 30-year-old, that is 152–181 bpm during the work. The dose is 2–3 sessions a week, 15–25 minutes each, with 48 hours between hard sessions. The energy cost is about 12–15 kcal per minute during work intervals for a 70 kg person, and the afterburn (EPOC) adds roughly 6–15% of the session total — about 20–40 kcal, not hundreds.
Worked example — a 30-year-old beginner doing a cycling HIIT session. Maximum heart rate estimate: 220 − 30 = 190 bpm. Target work intensity: 80–90% = 152–171 bpm. Session: 5-minute warm-up, then 8 rounds of 30 seconds hard (target 160 bpm) and 90 seconds easy, then a 5-minute cool-down. Total: 21 minutes, of which 4 minutes is hard work. Calories: about 200–250 kcal for the session, plus about 25 kcal of afterburn. After four weeks, the rest shortens to 60 seconds; after eight weeks, the work lengthens to 45 seconds. The progression is by shortening rest and lengthening work, not by adding sessions — two to three a week is enough.
Your plan, step by step
- Build a base first: four weeks of steady cardio (30 minutes, 2–3 times a week) before starting HIIT.
- Warm up for 5–10 minutes, then do 6–10 intervals of 20–60 seconds at 80–95% of maximum heart rate.
- Rest long enough to repeat the effort: equal rest for beginners, shorter rest as fitness improves.
- Do HIIT two to three times a week, on non-consecutive days, with easier days in between.
- Cool down for 5 minutes and stretch lightly — and stop if you feel chest pain, dizziness or severe breathlessness.
How to use this guide
Use HIIT as a time-efficient fitness tool, not as a daily punishment: two to three sessions a week, genuinely hard intervals, full recovery between them, and steady cardio or lifting on the other days. Judge it by fitness — a lower heart rate at the same pace, a faster recovery between intervals — over six to eight weeks. If you dread every session, the intensity or the frequency is too high; reduce one of them before you quit.
Common mistakes
Intervals that are not intense enough. If you can hold a conversation during the work interval, it is not HIIT — it is moderate interval training, which is fine, but it is not the same stimulus. The work should feel hard (8–9 out of 10), and the heart rate should reach 80–95% of maximum.
Rest periods that are too short. Rest that is too short means the next interval is slower, the intensity falls, and the session becomes steady cardio. Rest long enough to repeat the effort — equal rest for beginners, and shorten it only as fitness improves.
Doing HIIT every day. HIIT is a hard stress that needs 48 hours of recovery, like heavy lifting. Daily HIIT causes under-recovery, injury and burnout. Two to three sessions a week is the right dose for most people.
What this guide does NOT tell you
HIIT guidance on this page is for healthy adults; people with heart conditions, high blood pressure, asthma, joint problems or other medical conditions should discuss HIIT with a clinician before starting, and beginners should build a base of steady cardio first. Stop immediately and seek medical help if you feel chest pain, severe breathlessness, dizziness or faintness during exercise. This guide is general information, not medical advice.
Frequently asked questions
What is HIIT?
High-intensity interval training: repeated bouts of 20–60 seconds at 80–95% of maximum heart rate, separated by recovery, for a total of 15–25 minutes. The work must be genuinely hard (8–9 out of 10 effort) and the rest long enough to repeat it. HIIT improves cardiovascular fitness as much as much longer steady exercise in a fraction of the time — but it is demanding, and it needs 48 hours of recovery between sessions.
How many calories does HIIT burn?
About 12–15 kcal per minute during the work intervals for a 70 kg person, so a 20-minute session burns roughly 200–300 kcal. The 'afterburn' (EPOC) adds about 6–15% of the session total — 20–40 kcal, not the hundreds that marketing claims. HIIT is efficient because it is intense, not because it burns calories for hours afterwards.
How often should I do HIIT?
Two to three times a week, on non-consecutive days, with easier days in between. HIIT is a hard training stress that needs 48 hours of recovery, like heavy lifting. Daily HIIT causes under-recovery, injury and burnout, especially for beginners. Two to three sessions is the right dose for most people.
Is HIIT better than steady cardio?
For improving fitness in limited time, yes — HIIT improves VO2 max by 10–15% in 6–8 weeks, similar to or better than steady training in half the time. For fat loss, the difference is small when total calories are matched, because the session is shorter. For joint health and for beginners, steady cardio is gentler. The best programme uses both: HIIT twice a week and steady cardio on other days.
Can beginners do HIIT?
Yes, with a base first: four weeks of steady cardio (30 minutes, 2–3 times a week), then start with longer intervals at a lower intensity — 30 seconds on, 90 seconds off, at 70–80% of maximum heart rate — and progress over weeks. People with heart conditions, high blood pressure or joint problems should discuss HIIT with a clinician first.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.