Heart Rate Zone Calculator

Zone 1 (50-60% of reserve) is recovery. Zone 2 (60-70%) builds aerobic base and fat oxidation. Zone 3 (70-80%) is aerobic endurance. Zone 4 (80-90%) sits around lactate threshold. Zone 5 (90-100%) develops maximal aerobic power and can only be held briefly.

Because it builds mitochondrial density and capillary networks with relatively little fatigue cost, so you can accumulate a lot of it. Elite endurance athletes typically spend around 80% of training time at this intensity, which is the origin of the polarised training model.

A common distribution is roughly 80% in zones 1-2 and 20% in zones 4-5, with little deliberate time in zone 3. Sessions in zone 5 are measured in minutes, not hours — a few intervals is a full workout.

At steady, moderate intensity they are usually acceptable. During intervals, weights or anything with wrist movement they lag and can misread badly — often locking onto cadence rather than pulse. A chest strap is markedly better whenever precision matters.