How Heart Rate Zones Are Calculated

Heart rate zones appear on almost every fitness device and are presented with more precision than the underlying measurement supports. The estimate they rest on is unusually rough.

The zones are shares of a maximum

Zone systems divide the range between resting and maximum heart rate into bands, each associated with a different balance of fuel use and effort.

Lower zones correspond to efforts sustainable for a long time, while upper zones correspond to work that can only be held for minutes.

Everything therefore depends on the maximum figure, since it sets the ceiling from which every band is calculated.

Maximum heart rate is usually estimated, not measured

Devices apply an age-based formula, typically subtracting age from a fixed number, to produce a predicted maximum.

These formulas describe an average across a population and carry a wide spread around it, with individuals commonly differing by a substantial margin in either direction.

Someone whose true maximum sits well above the prediction will be told they are working harder than they are, and someone below it will be told the opposite.

Maximum heart rate is not a fitness measure

It is largely determined by age and genetics and barely changes with training, which is why it cannot be improved and should not be a target.

Training instead changes how much work is done at a given heart rate, and how much blood is pumped per beat.

This is why resting heart rate tends to fall with training while maximum stays put, and why comparing maximums between people says nothing about fitness.

Many things shift heart rate at a fixed effort

Heat, dehydration, altitude, caffeine, illness, poor sleep and stress all raise heart rate for the same workload.

Heart rate also lags behind effort, taking minutes to stabilise, which makes it unreliable for short intervals and for anything that changes quickly.

Wrist-based optical measurement adds further error during activities involving grip or rapid arm movement, where a chest strap is considerably more reliable.

What zones are actually good for

Their real value is preventing easy sessions from becoming moderately hard, which is the most common training error in endurance work.

Effort perception and the ability to hold a conversation track training intensity well and require no equipment or calibration.

Anyone with a heart condition, or on medication affecting heart rate, should have their training intensity guided by a clinician rather than a device formula.