It is a population mean with a spread of roughly ±10 beats, so for about a third of people it is out by more than 10 — and every training zone moves with it.
The formula was never properly derived, and it falls a full beat per year where the best-evidenced curve (Tanaka, from 351 studies) falls 0.7. The two cross at <b>exactly age 40</b>, so 220 − age reads high for the young and low for the old. Worse, the same percentage of your <i>maximum</i> and of your <i>reserve</i> are different heart rates — they differ by resting × (1 − percentage), about 24 beats at the bottom of zone 2 — which is why a watch and a coach can disagree about what easy means.
Estimate with the rule, then check it against the calculator that models it properly.
Open Heart Rate Zones & Max HR →It is a population mean with a spread of roughly ±10 beats, so for about a third of people it is out by more than 10 — and every training zone moves with it. The formula was never properly derived, and it falls a full beat per year where the best-evidenced curve (Tanaka, from 351 studies) falls 0.7.