VO2max

Apple Watch VO2max: accuracy, levels and how to raise it

Apple Watch shows your VO2max as Cardio Fitness. Where to find it, what the four levels mean, how accurate it is and why it can read low.

VO2max Health editorial team
· 5 min read

Key takeaways

1

Apple Watch estimates VO2max as Cardio Fitness from heart rate during outdoor walks, runs and hikes, and sorts it into Low, Below Average, Above Average or High for your age and sex

2

Apple's own validation was close on average, but two independent lab studies found it underestimated VO2max by about 6 ml/kg/min with errors around 13%

3

Heat, hills, heavy loads, wrong profile data and unlogged heart-rate medication all shift the number, so watch the trend over months

Apple does not call it VO2max on the screen. In the Health app it is Cardio Fitness: an estimate of your VO2max in ml/kg/min, worked out from how hard your heart works while you walk, run or hike outdoors. Here is where to find it, what the levels mean, how accurate it is and how to move it up.

How to measure VO2 max on Apple Watch

According to Apple Support, Apple Watch Series 3 or later records an estimate using the heart and motion sensors during an Outdoor Walk, Outdoor Run or Hiking workout in the Workout app. Indoor workouts do not count. The watch also uses your age, sex, weight, height and any medications that affect heart rate, and supports a range of 14–65 ml/kg/min.

Apple's technical paper adds the conditions: relatively flat ground (less than a 5% grade), good GPS and heart-rate signal, and enough effort to raise your heart rate by about 30% of the range between resting and maximum. You do not need to go flat out. Your first workout will not produce an estimate, and you need at least a day of wear first.

Where to find VO2 max on Apple Watch

Open the Health app on your iPhone, tap Browse, then Heart, then Cardio Fitness. Tap D, W, M, 6M or Y for different time spans, and Show All Cardio Fitness Levels to compare yourself with others of your age and sex.

Apple Watch cardio fitness levels

Apple sorts your estimate into one of four ranges: Low, Below Average, Above Average or High. Classifications are for people aged 20 and older and are based on the FRIEND registry of lab treadmill and bike tests. For a fuller set of reference values, see our VO2max chart and VO2max norms by age and sex.

Cardio fitness low on Apple Watch

If your level falls into Low, the watch can send a low cardio fitness notification, and another every 4 months if it stays low. Turn this on in the Apple Watch app on your iPhone: My Watch > Apps > Heart > Cardio Fitness Notifications. Apple's paper sets the threshold at the lowest fifth of the FRIEND data for ages 20–59; from 60, it uses fixed values of 18 ml/kg/min for men and 15 for women, levels linked to independent living.

A low reading can be real, or it can be distorted. Apple lists several reasons for underestimates: extreme heat, dehydration, caffeine, a recent move to high altitude, carrying a heavy backpack or a child, walking on sand, and pushing a stroller. A wrong age, sex or weight in your Health profile skews every estimate. The opposite problem also exists: if you take a heart-rate-limiting drug such as a beta blocker and do not record it in Health Details, the watch overestimates your VO2max.

VO2 max Apple Watch accuracy

Here is what the main validation data show.

Study People Compared with Main result
Apple technical paper, 2021 221 (validation group) VO2max projected from submaximal lab tests Average error 1.4 ± 4.7 ml/kg/min
Lambe et al., PLOS One, 2025 30 adults Maximal treadmill test with gas analysis Underestimated by 6.07 ml/kg/min; error 13.3%
Lambe et al., Mayo Clin Proc Digit Health, 2026 (Series 10) 35 young adults Maximal test with gas analysis Underestimated by 6.25 ml/kg/min; error 13.2%; only 14% placed in the right fitness band

Apple's reference was itself an estimate from submaximal tests, while the independent studies measured oxygen directly at maximal effort. That likely explains part of the gap. The broader picture matches the 2022 INTERLIVE meta-analysis: watches that use exercise data are reasonable for groups but can be well off for one person. Read more in how accurate is your watch's VO2max estimate?

How to increase VO2 max on Apple Watch

Two different goals: raising your real fitness, and getting a reading you can trust.

Raise your fitness

  1. Add one or two interval sessions a week. Apple's paper notes that high-intensity interval training gives the largest gains, generally about 5–10% over 6–12 weeks. Try the Norwegian 4×4, or Japanese interval walking if you are starting from walking.
  2. Build an easy base with zone 2 training. The heart rate zone calculator gives you the numbers.
  3. Stay consistent. Apple's paper also notes that fitness can fall by up to 27% within 2–3 weeks of inactivity.

Get a steadier reading

  • Log frequent Outdoor Walk, Outdoor Run or Hiking workouts, on flat routes, with good GPS.
  • Push the effort a bit higher in some sessions; Apple says higher exertion improves accuracy.
  • Wear the watch through the day, not only during workouts.
  • Keep your weight, age and medications up to date in the Health app.

Apple Watch vs Garmin VO2max

Both use the same idea, heart rate against speed, but differ in the details.

  • Activities: Apple uses outdoor walks, runs and hikes. Garmin uses outdoor runs with GPS and, with a power meter, rides.
  • Effort: Garmin asks for at least 10 minutes at 70% or more of maximum heart rate. Apple works at lower effort, which suits walkers.
  • Labels: Apple has four levels from FRIEND data; Garmin has five colour-coded levels from Cooper Institute data. See Garmin VO2max explained.

Neither replaces a lab test. If you want a measured number, see VO2max tests explained. You can also cross-check with our VO2max calculator, the VO2max running calculator or the fitness age calculator.

Apple, Apple Watch and Garmin are trademarks of their respective owners. VO2max Health is independent and not affiliated with or endorsed by any of them.

Frequently asked questions

Is Apple Watch VO2 max accurate?

In Apple's validation the average error was small, about 1.4 ml/kg/min. Independent lab studies in 2025 and 2026 found it underestimated VO2max by about 6 ml/kg/min on average, with a typical error of about 13%. It is better for tracking your own trend than for an exact number.

Why is my cardio fitness low on Apple Watch?

Anything that raises your heart rate at a given walking or running speed lowers the estimate: heat, dehydration, caffeine, a recent move to altitude, hills, sand, a heavy backpack or pushing a stroller. Wrong age, sex or weight in the Health app also skews it. If it stays low, more regular aerobic exercise is the main fix.

How do I increase my VO2 max on Apple Watch?

Exercise more often and harder. Intervals raise VO2max most, typically by about 5 to 10% over 6 to 12 weeks. For a more reliable reading, log frequent Outdoor Walk, Outdoor Run or Hiking workouts on flat ground and wear the watch all day.

Where do I find VO2 max on Apple Watch?

Open the Health app on your iPhone, tap Browse, then Heart, then Cardio Fitness. Tap Show All Cardio Fitness Levels to see how your number compares with others of your age and sex.

Sources

#wearables#vo2max#research#training

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