How the zones work
Everything here is built on one estimate: your maximum heart rate. Training zones are percentages of that maximum, or — more usefully, if you supply your resting heart rate — percentages of your heart-rate reserve.
Why this calculator asks your sex
Most calculators use 220 − age for everyone. That formula was derived from samples that were predominantly male, and it does not describe women well.
The St. James Women Take Heart Project followed 5,437 asymptomatic women for around 16 years and measured peak heart rate directly during maximal stress testing. It found mean peak heart rate ≈ 206 − 0.88 × age, and concluded plainly that "the traditional male-based calculation overestimates the maximum HR for age in women".
The gap matters in practice. A 40-year-old woman gets an estimated maximum of 180 bpm from the old formula but about 171 bpm from the women-specific one — and because every zone is a percentage of that number, a 9 bpm error at the top propagates all the way down. Someone aiming for easy zone-2 work using the male formula can end up training harder than they intend, which is precisely the opposite of what zone 2 is for.
So this calculator applies 206 − 0.88 × age when you select female and 220 − age when you select male, and tells you which one it used.
- Zone 1 (50–60%) — very light. Warm-ups, cool-downs, active recovery.
- Zone 2 (60–70%) — easy, conversational. You could hold a conversation in full sentences. The backbone of endurance training.
- Zone 3 (70–80%) — moderate aerobic. Talking becomes clipped.
- Zone 4 (80–90%) — hard. Sustainable for perhaps 20–60 minutes; improves lactate threshold.
- Zone 5 (90–100%) — maximal. Short intervals only.
Both formulas remain population averages with wide individual scatter — genuinely measured maximums vary by roughly ±10–12 bpm around either prediction. If you train seriously, a proper field test or lab test beats any equation. Treat these numbers as a starting range, not a verdict.
The Karvonen method
Taking percentages of maximum heart rate ignores where your heart actually starts from. The Karvonen method fixes this by working from your heart-rate reserve — the gap between resting and maximum:
Target = ((HRmax − HRrest) × intensity%) + HRrest
Worked example: a 40-year-old with a resting heart rate of 60. Estimated maximum is 220 − 40 = 180, so reserve is 180 − 60 = 120. For 70% intensity: (120 × 0.70) + 60 = 144 bpm. By contrast, a plain 70% of maximum gives 126 bpm — an 18-beat difference, and the Karvonen figure is the better target for a fitter person.
This matters most for people at the extremes of fitness. A well-trained athlete with a resting heart rate of 45 and a sedentary person at 80 will get very different, and more appropriate, targets from Karvonen than from percentage-of-max.
Why 220 minus age is a weak formula
It is worth being blunt: this equation is popular because it is memorable, not because it is accurate. It was never derived from a rigorous study — it emerged from a rough fit to pooled data in the 1970s — and it carries a standard deviation of roughly 10–12 beats per minute.
In practice that means a substantial share of 40-year-olds have a true maximum anywhere between about 160 and 200, against the 180 the formula predicts. Alternative equations such as 208 − (0.7 × age) fit the population data somewhat better, particularly for older adults, but none escapes the fundamental problem: individual variation in maximum heart rate is large and largely genetic.
The only way to know your actual maximum is a supervised maximal exercise test. For most people that is unnecessary — which is why the perceived-effort check below is worth more than the arithmetic.
Use perceived effort as a cross-check
The talk test is free, requires no device, and is surprisingly robust. If you can speak in full sentences comfortably, you are in Zone 2. If you can manage short phrases only, you are around Zone 3 to 4. If you can barely get a word out, you are in Zone 5.
When your monitor and your body disagree, trust your body. Heart rate is also affected by heat, dehydration, caffeine, illness, stress, and poor sleep — all of which can raise it by ten beats or more at the same workload, which is worth knowing before you conclude you have lost fitness overnight.
Which zone should you actually train in?
Most endurance athletes follow something close to a polarised distribution: the large majority of training volume easy, in Zone 2, with a small proportion genuinely hard in Zones 4 and 5. The common error among recreational exercisers is the opposite — spending most sessions in Zone 3, which is too hard to accumulate volume comfortably and too easy to drive top-end adaptation.
On the "fat-burning zone": it is real but widely misunderstood. Lower intensities do use a higher percentage of fat for fuel, but higher intensities burn more total calories and often more total fat per minute. For body composition, total energy expenditure and diet matter far more than which fuel your muscles preferred during the session.
Limitations and safety
Every number here rests on an estimated maximum heart rate carrying a 10–12 beat standard deviation, so treat the zone boundaries as broad regions rather than lines. Beta blockers and some other medications directly lower heart rate and make these calculations inapplicable.
If you experience chest pain, unusual breathlessness, dizziness, or an irregular heartbeat during exercise, stop and seek medical advice. Anyone with a known heart condition, or who is new to vigorous exercise and has cardiovascular risk factors, should speak to a doctor before training at high intensities.