What a zone actually is
A heart rate zone is a band of intensity, defined by how close you are to your own ceiling. The point of naming the bands is that different physiological adaptations happen in different parts of the range, and that the effort which feels most virtuous is rarely the one that produces the most fitness.
Two things have to be established before any band can be drawn: the top of the range, and how the range is divided.
maximum heart rate — the ceiling
resting heart rate — the floor
heart rate reserve — the distance between them
The ceiling is the hard part, and everything downstream inherits its error.
Where 220 minus age came from
It came from nowhere in particular. That is not a rhetorical flourish — the equation has no original derivation in the literature. It appeared in the early 1970s as a rough eyeball fit through data gathered for other purposes, was printed in a widely read source, and became the default by repetition rather than by evidence.
It is not useless. It is simply less accurate than an alternative that costs nothing to use instead:
220 − age the traditional figure
208 − 0.7 × age Tanaka, Monahan & Seals (2001)
Tanaka's version came out of a meta-analysis of the published studies and a laboratory validation. The two agree closely in the mid-30s and diverge at both ends of the age range: the traditional equation over-estimates in younger people and under-estimates in older ones, which is exactly where a training prescription based on it goes wrong.
For a 25-year-old the traditional equation sits about five beats higher than Tanaka's, so zones built on it are set slightly too hard. At 65 it sits about seven and a half beats lower, so they are set too easy. Both errors matter most to the people least able to notice them.
Both are estimates of a population, not measurements of a person. The standard deviation between individuals is around 10 to 12 beats per minute. For a 40-year-old with a predicted maximum of 180, a true maximum of 165 or 195 is entirely ordinary. That spread is wider than most of the zones themselves, which is the single most important thing on this page: your zones may be a whole band out, in either direction, and nothing about the arithmetic will reveal it.
Percentage of maximum, or heart rate reserve
Two conventions divide the range, and they do not give the same numbers.
Percentage of maximum takes a share of the top figure alone. Simple, and what most published charts and gym posters use. Its flaw is that it ignores the floor: at 50% of a 180 maximum, the target is 90 bpm, which for someone with a resting rate of 75 is barely above sitting still.
Heart rate reserve, the Karvonen method, published in 1957, works from the distance between resting and maximum:
target = resting + (maximum − resting) × intensity
Applied to the same person: resting 60, maximum 180, reserve 120. Zone 2 at 60-70% of reserve gives 132-144 bpm, where the percentage-of-max method gives 108-126. Substantially higher, and generally closer to how the effort actually feels.
Karvonen zones have a second advantage: they respond to training. As conditioning improves the resting rate falls, the reserve widens, and the zones shift with it. Percentage-of-max zones never move until you have another birthday.
Both are on the tool above. If a resting heart rate is available, reserve is the better basis; if not, percentage of maximum is what remains.
Measuring rather than predicting
Any measured maximum beats any equation, and there are three honest ways to get one.
From a hard effort you have already done. Look at the highest figure your monitor recorded during a genuinely maximal effort — the last minutes of a race, or a long hill climbed as hard as you could hold. Ignore obvious artefacts: a single spike to 210 in the first minute of a run is a strap contact error, not a heart rate.
A field test. Various protocols exist; most involve several minutes of building effort followed by an all-out finish. They demand real motivation, they only work if you are already fit enough to push safely, and they are unpleasant.
A clinical test. A supervised maximal exercise test gives the most reliable figure and is the option worth taking if there is any medical reason for caution.
The resting rate is easier and more useful than most people expect. Measure it before getting out of bed, over several mornings, and average. Its trend matters more than its value: a jump of five or more beats above your own baseline, held for a few days, is a fatigue, illness or sleep signal — and it shows up before performance does.
The training logic behind the bands
The bands are a convention, not a physiological fact — different coaching systems draw them in different places, and the adaptations blend into one another. Broadly:
Zones 1 and 2 develop the aerobic base: capillary density, mitochondrial content, fat oxidation. This work is sustainable, recoverable, and where most endurance volume belongs. It also feels too easy, which is why it is so consistently skipped.
Zone 3 is the moderately hard middle. It feels productive and it is not without value, but it produces less adaptation than zone 2 for the same time invested while costing considerably more recovery. Training that drifts here without meaning to is the most common pattern in recreational endurance sport.
Zones 4 and 5 raise threshold and maximal capacity. Effective in short doses with real recovery around them, and quick to accumulate into overreaching if repeated too often.
The distribution used by most endurance programmes is roughly 80% of time low and 20% high, with little deliberately in the middle. The polarised pattern is well described in the coaching literature; the practical difficulty is not knowing it but keeping the easy days genuinely easy.
Where heart rate is the wrong instrument
Heart rate is a response, not an input, and it has known lags and distortions.
It lags effort by a minute or two. For intervals shorter than about three minutes the number never catches up with what you are doing. Pace or power is the right measure there.
It drifts upward at constant effort. Cardiac drift of five to ten beats over a long steady session is normal — from rising core temperature and falling plasma volume, not from working harder. Chasing the original number by slowing down is usually the wrong response.
Heat, altitude, dehydration and stress all raise it for the same work. A session that feels correct at 155 in March may sit at 165 in July at identical effort.
Fatigue can push it either way. Under-recovery often shows as an elevated resting rate, and deep fatigue can suppress the maximum reachable — so a day when the rate refuses to climb is not necessarily a day to push harder.
Medication changes everything. Beta blockers and other rate-limiting drugs lower both the resting and maximum rate, and no age-based prediction survives them. The same applies with a pacemaker or a diagnosed arrhythmia: zones should come from the clinician managing the condition, not from a formula. Nothing on this page is medical advice, and chest pain, unexplained breathlessness or fainting during exercise is a reason to stop and get assessed rather than a reason to adjust a zone.