Fat burning heart rate zone: what it really measures, and what burns fat

What the fat burning zone actually is

During exercise, the muscles burn a mix of fat and carbohydrate. At low intensity, a large share of the energy comes from fat, but the total amount of energy used is small. As intensity rises, total energy use climbs, and the share coming from carbohydrate climbs faster. Somewhere in the middle, the amount of fat burned per minute reaches a peak: that intensity is Fatmax.

A classic study by Romijn and colleagues, with tracers in trained subjects, showed the pattern clearly: the release of fat from fat tissue was highest at the lowest intensity tested, while the use of glucose and muscle glycogen rose with intensity. Achten's study then defined Fatmax and a Fatmax zone, the range of intensities where fat burning stays within 10% of its peak. Above that zone, fat burning dropped sharply, and it became negligible at around 92% of maximum heart rate.

Where the zone sits, and why it varies

The two main studies give different averages, which is itself the lesson.

• Achten and colleagues: 18 moderately trained cyclists. Fatmax averaged 64% of maximal oxygen uptake, equal to 74% of maximum heart rate. The zone spread from about 55% to 72% of maximal oxygen uptake.
• Venables and colleagues: 300 healthy men and women on a treadmill. Fatmax averaged 48% of maximal oxygen uptake, equal to 61.5% of maximum heart rate. Women burned more fat than men and reached Fatmax at a higher intensity.

In Venables' study, sex, fitness and activity level together explained only 12% of the differences in fat burning between people. Body fat did not predict it at all. A single number printed on a cardio machine cannot capture that variation. Only a laboratory test with gas analysis measures your own Fatmax; a heart rate range is an estimate.

Why the zone does not decide fat loss

The fat burning zone answers the question of which fuel you use during the session. Fat loss depends on a different question: how much energy you use over days and weeks, compared with what you eat.

A harder session burns fewer fat calories per minute but more calories in total. That is why the zone alone cannot predict fat loss, and why trials comparing intensities matter more than fuel measurements. Wewege's meta-analysis of 13 trials in adults living with overweight or obesity found that high-intensity interval training and moderate continuous training both reduced total body fat and waist circumference, with no significant difference between them; the interval sessions took about 40% less time. Interestingly, the effects came mostly from running; cycling programmes did not reduce fat mass in that analysis.

The amount of exercise matters more than its zone. The American College of Sports Medicine found that 150 to 250 minutes a week produces modest weight loss, and that more than 250 minutes a week brings clinically significant loss. Exercise also reduces fat around the organs even when the scale barely moves: a meta-analysis by Verheggen and colleagues found that exercise lowered visceral fat by about 6% without weight loss. Our guide to losing belly fat covers this in detail.

How to calculate your range

The calculator uses two classic formulas. Tanaka's equation estimates maximum heart rate as 208 minus 0.7 times your age, and Karvonen's method uses your heart rate reserve, the gap between your maximum and your resting pulse, to set a range that fits your fitness.

1. Measure your resting heart rate in the morning, before coffee, sitting quietly for a few minutes.
2. Enter your age and resting pulse. The calculator gives your estimated maximum, a moderate range at 60 to 70% of heart rate reserve, and a hard range for intervals.
3. Check it with the talk test. In the moderate range you should be able to talk in full sentences, but not sing.

The moderate range overlaps the upper part of what the studies found: Fatmax sat at 74% of maximum heart rate in Achten's cyclists and at 61.5% in Venables' larger group. It is the same range as zone 2 training. A formula gives the average for people of your age, and your own maximum may differ, so trust your breathing as much as the number. For more on the formulas, see the max heart rate calculator.

How to use the zone in a week

• Long, easy sessions: brisk walks, cycling or jogging in the moderate range, 30 to 60 minutes. Comfortable, sustainable, and easy to recover from.
• One or two harder sessions: intervals for fitness and time efficiency. Our guide to short, hard workouts explains the dose.
• Two strength sessions: the WHO recommends muscle-strengthening activity on at least two days a week, for every adult, whatever the goal.
• The total: the WHO recommends 150 to 300 minutes of moderate activity a week, or 75 to 150 of vigorous activity, for all adults.

The best zone for fat loss is the one that lets you train often enough, for long enough, without dreading the next session.

Heart rate formulas do not apply if you take medicines that slow the heart, such as beta blockers, or if you have a heart condition. If you are starting from very little activity, live with obesity, diabetes or high blood pressure, or feel chest pain, dizziness or unusual breathlessness during exercise, see your doctor before training by heart rate.

Put the zone to work with Never Skip Club

A heart rate range only helps if the sessions actually happen. In Never Skip Club, you choose the body area, the level (Easy, Medium or Intense), 20, 30 or 45 minutes and where you train, and the app builds a guided session of five or six animated exercises in about a minute, with your sets logged. Start a crew of three to seven friends with a rhythm such as four sessions a week for a month: each session of 30 minutes or more counts, and the ranking rewards regularity, not extra sessions, which is exactly what fat loss needs. Create your free account, and if you want an expert to build your week around your heart rate, find a coach from our network. Already a member? Sign in.