How Apple Watch Calculates Your Max Heart Rate (HRmax)
Apple has never published the formula. Garmin says it uses Tanaka, COROS says it recommends Gellish, and Apple says this: "Heart Rate Zones are a percentage of your maximum heart rate and are automatically calculated and personalized using your health data." That is the entire public specification. There is no HRmax field in the Health app or the Watch app to read or edit, which makes Apple the one major brand where you cannot see the number your zones are built on.
What Apple actually says
The stated model is %HRmax, and "personalized using your health data" implies recorded heart rates rather than age alone. Apple does not say which data, over what window, or how it is weighted.
One more documented sentence carries the rest: "Heart Rate Zones are calculated only if you've entered your date of birth in the Health app on your iPhone."
That is the tell. If zones will not compute without a date of birth, age is load-bearing in the estimate. Apple will not say what the age term is. There are five zones, described as effort levels from easier to harder.
What the community has worked out
None of this is confirmed by Apple. Treat it as the best available theory.
The reverse-calculation anomaly. On Apple's own support forums, users take the zone boundaries their watch displays and work backwards to the maximum heart rate those boundaries imply. The numbers land nowhere near 220 − age. One user whose age predicts 167 bpm reported implied maximums ranging from 177 to 207 depending which boundary they used, and described the app as stretching the top of the zones and pushing the bottoms up.
The heart rate reserve theory. The explanation offered in that thread and by third-party trackers is that Apple is not using plain %HRmax at all. It is using heart rate reserve, the Karvonen method, which works from resting heart rate and maximum and places each zone in the gap between them. One published account adds that Apple refreshes both numbers at the start of each month from the previous month's recorded data.
The arithmetic fits. Take that user: a predicted maximum of 167 and a resting heart rate of 50 gives a reserve of 117 beats. Under Karvonen the 60% boundary is 50 + 0.6 × 117, so 120 bpm. Back that out as a plain percentage of maximum and you get 120 ÷ 0.6 = 200. Do it at the 80% boundary and you get 180. Reserve-based zones, read as %HRmax zones, produce exactly the inflated and inconsistent maximums people report.
Two things follow if that is right. The zones are more personal than a bare age formula, because your resting heart rate is your own measured number. And your Apple "Zone 2" is not a Garmin "Zone 2", because they are percentages of different quantities, a gap covered in LTHR vs %HRmax zones. It also sits awkwardly with Apple's own wording, and Apple has not resolved the contradiction.
The fix: set zones 2, 3 and 4 by hand
Age-based HRmax estimates carry an error of roughly ±10 to 12 bpm for an individual. On a Garmin or COROS you fix that by typing a tested number in. On an Apple Watch there is nowhere to type it, so if the estimate is eight beats high, every boundary is eight beats high and nothing tells you. What Apple does give you is manual zones.
On the watch: Settings → Workout → Heart Rate Zones → Manual, then tap zone 2, 3 or 4 and enter limits.
On the iPhone: Apple Watch app → My Watch → Workout → Heart Rate Zones → Manual.
You can only edit zones 2, 3 and 4, which is less restrictive than it sounds. Zone 1 ends where zone 2 begins and zone 5 starts where zone 4 ends, so three bands set all five. Zone 5 has no ceiling.
The workflow is the reverse of every other brand. Instead of correcting HRmax and letting the watch derive zones, you derive the zones and hand Apple the answers.
Get a real HRmax first: build to hard over 10 to 15 minutes, finish with an all-out 60 to 90 second effort, take the highest reading off a chest strap, repeat within two weeks and keep the higher number. Not for anyone with cardiac risk or new to hard training. Full protocol in finding your maximum heart rate.
Then turn it into boundaries:
Maximum heart rate
185bpm
| Zone | Range | What it trains |
|---|---|---|
| Z1 | Recovery 93–111 bpm | Active recovery and warm-up; promotes blood flow without adding fatigue. Very easy, RPE 1–2. Full conversation, barely breathing hard. |
| Z2 | Endurance 111–130 bpm | Aerobic base, fat oxidation and capillary density, the bread-and-butter zone. Comfortable, RPE 3–4. Can hold full sentences. |
| Z3 | Tempo 130–148 bpm | Aerobic capacity and muscular endurance; 'moderately hard' steady work. Working, RPE 5–6. Short sentences only. |
| Z4 | Threshold 148–167 bpm | Lactate threshold, your sustainable race-pace ceiling. Hard, RPE 7–8. A few words at a time. |
| Z5 | VO₂max 167–185 bpm | Maximal aerobic power and VO₂max via short, hard intervals. Very hard to maximal, RPE 9–10. No talking. |
Z1 · Recovery. Active recovery and warm-up; promotes blood flow without adding fatigue. Very easy, RPE 1–2. Full conversation, barely breathing hard.
Z2 · Endurance. Aerobic base, fat oxidation and capillary density, the bread-and-butter zone. Comfortable, RPE 3–4. Can hold full sentences.
Z3 · Tempo. Aerobic capacity and muscular endurance; 'moderately hard' steady work. Working, RPE 5–6. Short sentences only.
Z4 · Threshold. Lactate threshold, your sustainable race-pace ceiling. Hard, RPE 7–8. A few words at a time.
Z5 · VO₂max. Maximal aerobic power and VO₂max via short, hard intervals. Very hard to maximal, RPE 9–10. No talking.
- Zones are simple percentages of maximum heart rate (Z1 50–60% up to Z5 90–100%). They are easy to set but assume your HRmax is accurate.
- Heart rate lags effort by 1–3 minutes and drifts upward in heat or dehydration ('cardiac drift'), so treat short, hard efforts by feel.
A PDF of your personalized results, with a QR code to reopen them anytime.
Type the zone 2, 3 and 4 figures into the watch. If you know your resting heart rate, Karvonen gives more personal boundaries, and a tested threshold heart rate makes LTHR zones better again.
FAQ
What formula does Apple Watch use for max heart rate? Apple does not publish one. It states only that zones are a percentage of your maximum heart rate, personalised from your health data, and that they will not calculate without your date of birth.
Can I set my max heart rate on an Apple Watch? No. There is no HRmax field. Override the zone boundaries instead, in Settings → Workout → Heart Rate Zones → Manual.
Does Apple Watch use heart rate reserve? Probably, according to community reverse-engineering, but Apple has never confirmed it. Users who back-calculate a maximum from their zone boundaries get inflated, inconsistent numbers, which is what reserve-based zones produce when read as percentages of maximum.
Does Apple Watch use 220 minus age? Apple has never confirmed it. Age is clearly involved, since zones need a date of birth, but the formula is unpublished and user reports are inconsistent.
Why can I only edit zones 2, 3 and 4? Those three bands define all five. Zone 1 ends where zone 2 starts, and zone 5 runs upward from the top of zone 4.
Other brands
- Garmin uses Tanaka and auto-raises HRmax from recorded peaks.
- COROS recommends Gellish and runs three zone models.
- Apple Watch publishes no formula at all.
- Polar, Suunto, Fitbit, Samsung and Strava all default to 220 − age.
- WHOOP learns it from your data. Wahoo skips HRmax and anchors to threshold.
Sources
- Apple Support: View Heart Rate Zones on Apple Watch
- Apple Support: Cardio fitness levels
- Apple Support Communities thread on zone calculation
- Training Today on Apple Watch zone accuracy
- How Garmin calculates your max heart rate
- How COROS calculates your max heart rate
Cover photo by iGeeksBlog, CC BY-SA 4.0, via Wikimedia Commons.
