HR Zones Calculator

The standard method estimates your maximum heart rate as 220 minus your age, then sets each zone as a percentage band of that maximum. If you also enter a resting heart rate, the tool uses the Karvonen method — Target HR = ((Max HR − Resting HR) × intensity%) + Resting HR — which personalises the zones to your fitness level, since a fitter person with a lower resting heart rate gets different targets than the age formula alone gives.

Zone 1 (50–60%) is recovery and warm-up. Zone 2 (60–70%) is your aerobic base — fat oxidation at a conversational pace. Zone 3 (70–80%) is tempo. Zone 4 (80–90%) is threshold, where conversation gets hard. Zone 5 (90–100%) is maximum effort, sustainable only in short bursts. Different zones train different systems, so a mix matters more than always pushing hard.

Zone 2 builds the aerobic base: at that conversational intensity the body preferentially burns fat and improves mitochondrial density and metabolic flexibility — the ability to switch efficiently between fuels. It is also sustainable enough to accumulate real volume without the recovery cost of harder efforts. For metabolic health specifically, low-intensity aerobic work improves insulin sensitivity, which is why Zone 2 gets singled out.

It is a population average with a wide individual spread — your true maximum can sit 10 to 20 beats above or below the estimate. That is fine as a starting point, but if you know your actual max from a field or lab test, use it. Entering a resting heart rate to trigger the Karvonen method also improves the personal fit, since it factors in your current fitness — pair the output with your TDEE to plan training volume against energy needs.

A widely used approach is polarised training: spend most of your time — roughly 80% — in the easy Zone 1–2 range, and a smaller share, around 20%, in the hard Zone 4–5 range, with relatively little in the moderate Zone 3 “grey zone.” Easy work builds the aerobic base cheaply while a little hard work drives the high-end adaptations; living in the middle gives you fatigue without the full benefit of either end.