How Heart Rate Zones Are Calculated on Apple Watch, Garmin, WHOOP and Fitbit, and Why Zone 2 Differs

Short answer: heart rate zones come from two choices, a formula for maximum heart rate and a rule for cutting zones from it, and every vendor makes those choices differently.

The result is that “zone 2” is not one number. For the same 45-year-old with a resting heart rate of 60, it lands anywhere from about 105 to 140 beats per minute depending on whether you ask Garmin, WHOOP or Fitbit. That matters to anyone who trains by zones, and even more to products that compare or reward time in zone across devices.

This post explains the two building blocks, how each major vendor calculates zones by default, why the formulas disagree, and what iOS 27 changed for developers who want to read zones instead of reinventing them.

What are the building blocks?

1. Maximum heart rate. Almost every default starts from an age formula.

• 220 minus age is the most common default, used by Garmin and Fitbit [1][2].
• 208 minus 0.7 times age comes from Tanaka and colleagues, who pooled 351 studies with 18,712 people and confirmed it in a lab study of 514 more. It gives higher maximums for older adults than 220 minus age, and did not differ between men and women or by activity level [3].
• 206 minus 0.88 times age comes from Gulati and colleagues’ study of 5,437 women, and predicts a lower peak heart rate for women than 220 minus age at most adult ages: about 166 against 175 at 45 [4].

All three describe averages. Individuals scatter widely around them, which is why a measured maximum from a hard effort or a lab test produces better zones than any formula.

2. How zones are cut.

• Percent of maximum heart rate. Zone 2 might be 60% to 70% of maximum. Simple, and blind to fitness: two people of the same age get the same zones.
• Percent of heart rate reserve, the Karvonen method. Reserve is maximum minus resting heart rate; a zone is a percentage of reserve added back to resting heart rate. A fitter person with a lower resting heart rate gets lower boundaries [5].
• Percent of lactate threshold heart rate. Anchored to a tested threshold rather than an estimated maximum, offered by Garmin as an option [1].

How does each vendor calculate zones?

Two details matter.

Garmin’s default ignores resting heart rate. Unless the user changes the setting, a trained cyclist and an untrained office worker of the same age get identical zones [1].

Apple does not publish its formula. Its support page says only that zones are automatically calculated and personalized using health data, which requires a date of birth [7]. Independent analyses describe a heart rate reserve approach that uses resting heart rate, but the exact maximum heart rate estimate is Apple’s own.

Why does zone 2 differ so much?

The worked example uses each vendor’s published method. Three things drive the spread.

• Maximum heart rate. 220 minus age gives 175 for a 45-year-old; Tanaka gives about 177; Gulati gives about 166 for a woman [3][4]. Every zone boundary moves with it.
• Reserve versus maximum. Percentages of reserve sit higher than the same percentages of maximum, because resting heart rate is added back. WHOOP’s 60% to 70% and Garmin’s 60% to 70% are different targets [1][5].
• Naming. Fitbit’s “fat burn” spans roughly what Garmin calls zones 2 and part of 3 [2].

None of these is wrong. They are different definitions. In the endurance training literature, “zone 2” usually means easy aerobic work below the first lactate threshold, which none of these age formulas measures. That is why coaches who care about precise zone 2 use a lab or field test rather than a default.

How do zones relate to activity guidelines?

The WHO and the American Heart Association recommend at least 150 minutes of moderate or 75 minutes of vigorous activity a week [6]. Heart rate reserve maps onto that well: the American College of Sports Medicine defines moderate intensity as 40% to 59% of reserve and vigorous as 60% to 89% [10].

Fitbit’s Active Zone Minutes are a direct implementation. A minute in the fat burn zone, from 40% of reserve, earns one Active Zone Minute; a minute in cardio or peak, from 60%, earns two. The default goal is 150 a week, which is the guideline expressed in heart rate terms [2][6].

That is a useful model for any product: anchor zones to reserve, and count vigorous minutes double, and a weekly target lines up with public health guidance.

What changed for developers in iOS 27?

Until this year, every training app kept its own zone model, and Apple’s zones lived only in the Workout app.

iOS 27 added HKWorkoutZoneConfiguration, HKWorkoutZone and HKWorkoutZoneGroup to HealthKit [8]. A configuration defines a complete set of zones for a quantity type such as heart rate; a zone group carries the configuration together with time in each zone for a workout. Apps can read a person’s preferred zones, whether they set them or the system generated them, and read time in zone from completed and live workouts. The rest of the September changes are in Apple’s September health announcements.

Health Connect has no zone type [9]. On Android, apps read heart rate samples within exercise sessions and apply their own zone definition. Garmin, WHOOP and Fitbit compute zones in their own apps; whether time in zone is available through each vendor’s API varies.

How should a product store and compare time in zone?

• Store the definition with the minutes. Time in zone is meaningless without the boundaries that produced it. Keep the method, maximum heart rate, resting heart rate and zone cut-offs with every workout.
• Never compare minutes across vendors directly. Thirty minutes in Garmin zone 2 and thirty minutes in WHOOP zone 2 can be different workouts. If you must compare, recompute from raw heart rate samples with one definition.
• Prefer reserve when resting heart rate is known. It adapts to fitness and lines up with intensity guidelines [5][6].
• Let users override. A measured maximum or a tested threshold beats any formula [3].
• Count intensity, not just zones. For users without a heart rate sensor, intensity from step cadence or logged workouts still maps to the same guidelines; see activity intensity.

The short version

Heart rate zones depend on a maximum heart rate formula and a rule for cutting zones [1][3][4][5]. Garmin defaults to percent of 220 minus age; WHOOP and Fitbit use heart rate reserve; Apple personalizes zones from health data without publishing the method [1][2][5][7]. For one 45-year-old, zone 2 spans roughly 105 to 140 bpm depending on the device. iOS 27 lets apps read Apple’s zones and time in zone directly [8]; Health Connect does not [9]. Store the zone definition with every minute, recompute from raw heart rate when comparing devices, and anchor targets to reserve so they line up with the 150-minute guideline [6].