Almost every heart rate zone calculator starts the same way: subtract your age from 220, call that your maximum, and slice it into five bands. It is the most widely repeated number in fitness, and for a large share of people it is wrong by enough to matter.
This is not an argument against training by heart rate. It is an argument for getting the anchor number right, because every zone you train in is derived from it.
Where 220 minus age came from
The formula is usually traced to work by Fox and Haskell in the early 1970s. It was not the product of a controlled study designed to produce a predictive equation. It was a line drawn through a scatter of data points collected from a modest number of subjects, several of them from studies that were not originally about maximum heart rate at all, and it was offered as a rough guide rather than a precise tool.
It survived because it is easy to remember. That is genuinely the main reason.
How wrong it is
The important number is not the formula's average error, which is small. It is the spread around that average. Across populations, the standard deviation of measured maximum heart rate around the 220-minus-age line sits at roughly 10 to 12 beats per minute.
Work through what that means. A 40-year-old is assigned a maximum of 180. One standard deviation covers roughly 168 to 192. Two standard deviations — which still leaves about one person in twenty outside the range — covers roughly 156 to 204.
So a 40-year-old with a true maximum of 195 who trains off 180 will spend their "easy" sessions well below the intensity they were aiming for, and will never reach the top zones at all. A 40-year-old with a true maximum of 165 will find zone 4 physically unreachable and may conclude they are unfit rather than that the formula does not describe them.
The error is also not random across the age range. The formula systematically underestimates maximum heart rate in older adults, which is why it is the group most likely to be handed training zones that are too easy.
Better formulas, and their limits
Several equations fit the data better than 220 minus age:
- Tanaka — 208 − (0.7 × age). Derived from a large meta-analysis, and notably more accurate for older adults.
- Gellish — 207 − (0.7 × age). Very similar in practice.
These are improvements, and the calculators here let you pick between them. But understand what has improved: the average is now better centred. The individual spread barely moves. A formula cannot know that you happen to have an unusually high or low maximum, because nothing in your age tells it.
Formulas are the right tool when you have no better data. They are not the right tool when you could get better data in an afternoon.
Finding your actual maximum
The gold standard is a graded exercise test in a lab, with the intensity ramped until you cannot continue. That is accurate and most people will never do it.
A field test gets you most of the way. The general shape is a thorough warm-up followed by a sustained, genuinely maximal effort — commonly repeated hard hill efforts, or a progressive build to an all-out finish — with the highest heart rate observed taken as your working maximum. Several protocols exist and any of them beats a formula.
Two honest caveats. First, a maximal test is hard and unpleasant, and it carries real cardiovascular risk if you have any underlying condition, so it belongs after a conversation with a doctor rather than before one. Second, most people do not push as hard as they think in a self-administered test, so a field maximum is usually a slight underestimate. That is still much closer than an equation.
One thing that does not work: taking the highest number your watch has ever shown. That figure is frequently a stray reading from a wrist optical sensor picking up cadence, or from a chest strap with a dry electrode at the start of a run. If your recorded maximum appeared for four seconds in the first minute of an easy jog, it is an artefact.
Heart rate reserve: the upgrade that costs nothing
Even with a good maximum, slicing it into straight percentages ignores the other end of the scale. Two people with the same maximum of 190 but resting rates of 45 and 70 do not experience 70% of maximum the same way.
The Karvonen method fixes this by working from heart rate reserve — the gap between rest and maximum:
Target = resting HR + (intensity % × (max HR − resting HR))
For the 190-max athlete resting at 45, 70% of reserve is 45 + 0.7 × 145 = 147. For the one resting at 70, it is 70 + 0.7 × 120 = 154. Straight percentage-of-maximum would have given both of them 133.
Heart rate reserve needs one extra number you can measure accurately and for free: your true resting rate, taken lying down before you get out of bed, ideally averaged over several mornings.
Threshold-based zones
Many coaches skip maximum heart rate entirely and anchor zones to lactate threshold heart rate instead, on the grounds that threshold is the physiologically meaningful boundary and is also the thing that moves as you get fitter.
The common field estimate is a 30-minute time trial run alone at a maximal sustainable effort, taking your average heart rate over the final 20 minutes as threshold. Zones are then set as percentages of that.
The practical advantage is that threshold responds to training. Your maximum heart rate barely changes with fitness — it drifts down slowly with age and that is about it. So zones anchored to maximum stay static while you get fitter, and zones anchored to threshold move with you.
What heart rate cannot tell you
Even with a perfect maximum and well-set zones, heart rate has structural limitations worth knowing before you trust it session to session.
It lags. Heart rate takes roughly 30 to 90 seconds to respond to a change in effort. For intervals shorter than about two minutes, it spends most of the interval catching up, and the number you see at the end reflects where you were, not where you are. Short intervals are better governed by pace, power or perceived effort.
It drifts. On a long steady effort at constant pace, heart rate rises — commonly by several percent over an hour or more. This is cardiac drift, driven largely by heat and progressive dehydration. Chase a fixed heart rate on a long run in the warm and you will find yourself slowing down steadily to hold it.
It responds to everything else in your life. Poor sleep, illness, caffeine, alcohol, altitude, heat, and psychological stress all move heart rate. A session that feels normal but runs ten beats high is information, and it usually means something other than "run faster".
Medication changes the scale entirely. Beta blockers in particular lower both resting and maximum heart rate substantially, which makes every formula and every percentage meaningless. If you take one, zone-setting is a conversation with your doctor, not a calculator.
What to actually do
If you want zones you can trust, in order of effort:
- Measure your resting heart rate properly over several mornings. Free, takes a week of remembering.
- Use heart rate reserve rather than straight percentage of maximum. Free, takes one extra input.
- Do a threshold field test and anchor zones there. One hard session, repeated every few months as you get fitter.
- Do a maximal field test, or a lab test if you have access and a reason.
And if you do none of those, use Tanaka rather than 220 minus age, treat the resulting zones as approximate, and pay attention when perceived effort disagrees with what the watch says. Your own sense of effort is a genuinely good instrument, and it does not need calibrating against a formula from the 1970s.