Heart Rate Zones

Heart rate zones are a way to turn one number into context.

If your heart rate is 142, the number alone does not say much. It depends on your age, your maximum heart rate, and sometimes your resting heart rate. A zone gives that number a label you can use during a workout.

Zonas uses five workout zones:

Zone	Label	Plain-language feel
Zone 1	Recovery	Very easy
Zone 2	Aerobic	Comfortable conversation
Zone 3	Steady	Controlled effort
Zone 4	Hard	Short phrases only
Zone 5	Peak	Very hard

These labels are intentionally plain. Zonas does not call Zone 2 a “fat-burning zone” or Zone 5 a magic performance zone. Those words imply more certainty than a general heart rate model can give.

How Zonas Calculates Zones

Zonas starts with maximum heart rate.

If you have entered your own maximum heart rate, the app uses that. If not, Zonas estimates it from age with the Tanaka formula:

HRmax = 208 - trunc(0.7 x age)

That trunc detail matters. The iOS app uses Swift Int() math, so it truncates instead of rounding.

Then Zonas chooses the zone engine:

Available data	Method
Max heart rate and resting heart rate	Karvonen / heart rate reserve
Max heart rate only	Percent of max heart rate
Custom boundaries	Your manual settings

When resting heart rate exists, Zonas uses heart rate reserve:

target heart rate = RHR + intensity x (HRmax - RHR)

That is the better default because it accounts for the space between rest and maximum. A person with a resting heart rate of 48 and a person with a resting heart rate of 72 can have the same estimated HRmax, but the same workout heart rate may not mean the same effort for both.

When resting heart rate is missing, Zonas does not guess it. It falls back to percent of max heart rate.

The Five Bands

The workout-zone bands are familiar 10-point bands:

Zone	Karvonen / HRR	Percent HRmax fallback
Zone 1	50-59% HRR	50-59% HRmax
Zone 2	60-69% HRR	60-69% HRmax
Zone 3	70-79% HRR	70-79% HRmax
Zone 4	80-89% HRR	80-89% HRmax
Zone 5	90-100% HRR	90-100% HRmax

Internally, Zonas turns those percentages into whole BPM ranges. Lower bounds are truncated. Upper bounds stop one beat before the next zone begins, except Zone 5, which ends at max heart rate.

So the ranges stay clean. A heart rate belongs to one zone, not two.

Zones Are For The Workout

Workout zones answer a practical question:

How hard is this session right now?

That is different from the weekly cardio goal. Public-health guidance is written around moderate and vigorous activity, not consumer Zone 1-5 labels. Zonas keeps those layers separate.

The workout screen uses Zone 1-5 because it is fast to read while moving. The weekly card uses moderate-equivalent minutes because that is the stronger general-health framework.

Both matter. They just answer different questions.