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 exactly age 40, so 220 − age reads high for the young and low for the old. Worse, the same percentage of your maximum and of your reserve 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.
Rate-controlling medication removes the basis entirely: on beta blockers both resting and maximum rate fall, so every formula overestimates the ceiling and sets every zone too high. A measured maximum from a hard effort beats all of them, including this correction.
At 40 yr the rule says 180 bpm and the exact answer is 180 bpm — within half a percent. It holds to within 5% up to 66.8 yr, then drifts.
The two agree almost exactly at 40 and pull apart in both directions, so the rule flatters the young and short-changes the old - the group most likely to be training to a heart-rate number in the first place.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
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.