Calculate my zones
How your maximum heart rate is estimated
Every training zone is a percentage of your maximum heart rate, so everything depends on that first number. The only way to measure it exactly is a supervised maximal exercise test, which most people will never do — so we estimate it from age, using one of three formulas.
Fox is the familiar one, published in 1971. It is easy to remember and roughly right in middle age, but it drifts at both ends of the range — it tends to overestimate for younger people and underestimate for older ones.
Tanaka, from a 2001 review of the research, is the better estimate for most adults and is noticeably more accurate over 40. At 50 the two agree closely: Fox gives 170 bpm and Tanaka 173. By 60 the gap widens to 160 against 166 — enough to shift every zone boundary.
Karvonen works differently. Rather than taking a flat percentage of maximum, it uses your heart rate reserve — the range between resting and maximum — which personalises the zones to your fitness. It needs an accurate resting heart rate, but it is the most individualised of the three.
How accurate are these estimates?
Worth being straight about this: age-based formulas describe populations, not individuals. The standard deviation on the 220 − age estimate is around 10 to 12 bpm, which means a substantial share of people sit more than 10 bpm either side of their predicted maximum. Two 40-year-olds of identical fitness can have genuinely different true maximums.
The practical consequence is that zones are guides rather than gates. If Zone 2 feels punishingly hard or suspiciously easy, trust your body over the arithmetic. Perceived effort and the talk test — can you speak in full sentences, short phrases, or not at all? — remain excellent cross-checks, and they cost nothing.
What each zone actually trains
| Zone | Intensity | What it develops |
|---|---|---|
| 1 — Recovery | 50–60% | Blood flow and recovery between hard sessions. Full conversation possible throughout. Also your warm-up and cool-down. |
| 2 — Fat burn | 60–70% | Aerobic base — capillary density, mitochondrial function, fat metabolism. The zone most endurance training should sit in. Conversation possible but takes effort. |
| 3 — Aerobic | 70–80% | Cardiovascular efficiency and stamina. Short sentences only. Comfortable enough to overuse, which is a common training mistake. |
| 4 — Threshold | 80–90% | Lactate threshold and speed endurance. A few words at a time. Sustainable for perhaps 20 to 40 minutes when trained. |
| 5 — Maximum | 90–100% | Peak power and VO₂ max. Short intervals only, no conversation. Demanding to recover from — use sparingly. |
The “fat burning zone” is widely misunderstood
Zone 2 is labelled fat burn on nearly every treadmill, and the label is half true in a way that misleads.
At lower intensities a greater proportion of the energy you use comes from fat rather than carbohydrate. That much is accurate. But proportion is not the same as amount. A harder session burns more calories per minute, and typically more fat in absolute terms, even though fat makes up a smaller share of the total.
Measuring your resting heart rate properly
If you use the Karvonen method, the accuracy of your zones depends on this figure, and most people measure it wrong by taking it mid-morning after coffee.
Take it immediately on waking, before you sit up. Count beats for 30 seconds at the wrist or neck and double it, or read it from a fitness tracker's overnight low. Repeat across three or four mornings and take the average, since a single reading is easily thrown off by a poor night or a disturbing dream.
A typical adult resting heart rate falls between 60 and 100 bpm. Well-trained endurance athletes often sit between 40 and 60. A falling resting rate over weeks and months is one of the clearest signs your cardiovascular fitness is improving — often visible before any change in performance.
Using zones in a real training week
The most consistently supported approach in endurance training is deceptively simple: keep the great majority of your volume easy, and make the hard sessions genuinely hard. Roughly 80 per cent in Zones 1 and 2, the remaining 20 per cent in Zones 4 and 5.
The trap is Zone 3. It feels productive, so it is easy to drift into for every session — too hard to build a proper aerobic base, too easy to drive real adaptation. Training that lives permanently in the middle tends to plateau.
For general health rather than performance, the NHS recommendation is 150 minutes of moderate activity a week — broadly Zones 2 and 3 — or 75 minutes of vigorous activity, plus strengthening exercise on two days.
Frequently asked questions
Sources and further reading
- Fox SM, Naughton JP, Haskell WL. Physical activity and the prevention of coronary heart disease. Annals of Clinical Research, 1971;3(6):404–432. — the origin of the 220 − age formula.
- Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 2001;37(1):153–156. — the review that produced the more accurate 208 − 0.7 × age equation.
- Karvonen MJ, Kentala E, Mustala O. The effects of training on heart rate: a longitudinal study. Annales Medicinae Experimentalis et Biologiae Fenniae, 1957;35(3):307–315. — the heart rate reserve method.
- Gellish RL, Goslin BR, Olson RE, et al. Longitudinal modeling of the relationship between age and maximal heart rate. Medicine & Science in Sports & Exercise, 2007;39(5):822–829. — the variability around age-based estimates.
- Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 2010;5(3):276–291. — the evidence behind the 80/20 distribution.
- NHS UK — Physical activity guidelines for adults aged 19 to 64. — the 150-minute weekly recommendation.