Is Apple Watch VO2 max accurate? What the validation studies found

Apple Watch VO2 max is a reasonable estimate for tracking your own trend, but it is not as accurate as a lab test. Two independent studies found that it underestimated lab-measured VO2 max by about 4.5 to 6 mL/kg/min on average, with an average error of roughly 13 to 16%. Individual readings can be off by more, in either direction.

Apple's own validation reported a smaller error, against a different reference. This article sets out what each study measured, why the watch can miss, and how to get the most reliable number from it.

How does Apple Watch estimate VO2 max?

Apple Watch estimates VO2 max, which Apple calls cardio fitness, from your heart rate response during outdoor walks, runs and hikes, together with your speed from GPS and your age, sex, height, weight and medications from the Health app. It does not measure oxygen. The estimate is based on submaximal effort, so you do not need to reach your maximum heart rate.

Conditions Apple lists for an estimate:

Requirement	Detail
Workout type	Outdoor Walk, Outdoor Run or Hiking. Indoor workouts and gym equipment do not count
Terrain	Relatively flat ground, less than 5% incline or decline
Signal	Adequate GPS and heart rate signal quality
Effort	Heart rate rising by about 30% of the range from resting to maximum
Wear time	Watch worn for at least a day before the first estimate
Range	14 to 65 mL/kg/min, Apple Watch Series 3 or later

Caption: Apple Support and Apple VO2 max white paper (May 2021), as of October 2026.

What did the independent studies find?

Both independent studies found that Apple Watch underestimated VO2 max compared with lab testing using gas analysis. Lambe and colleagues (2025) tested 28 adults and found a mean underestimate of 6.07 mL/kg/min. Caserman and colleagues (2024) tested 19 adults and found the watch averaged 41.4 mL/kg/min against 45.9 measured in the lab.

Study	People	Average gap vs lab	Average error	Other findings
Lambe et al., PLoS One 2025	28 analyzed	Watch 6.07 mL/kg/min lower	MAPE 13.31%, MAE 6.92 mL/kg/min	Limits of agreement −6.11 to 18.26 mL/kg/min
Caserman et al., JMIR Biomed Eng 2024 (Series 7)	19	Watch about 4.5 mL/kg/min lower	MAPE 15.79%, RMSE 8.85 mL/kg/min	Reliability (ICC 0.47) rated poor

Caption: MAPE = mean absolute percentage error, MAE = mean absolute error, RMSE = root mean square error.

The wide limits of agreement in the 2025 study matter most for individuals. They mean that, for a given person, the watch could read from about 6 mL/kg/min above to about 18 mL/kg/min below the lab value. Lambe and colleagues concluded that the estimates need further refinement before clinical use, while noting the watch's practical value.

Does accuracy depend on fitness level?

Probably. Caserman and colleagues concluded that, like other smartwatches, Apple Watch tends to overestimate VO2 max in people with poor fitness and underestimate it in people with excellent fitness. Most participants in the 2025 study (21 of 30 recruited) had excellent or superior fitness, which may help explain the large average underestimate. Both studies were small, so the pattern needs confirmation.

What did Apple's own validation find?

Apple's 2021 white paper reports a mean error of 1.4 ± 4.7 mL/kg/min in a validation group of 221 people, and a test-retest reliability (ICC) of 0.86. Apple's reference, however, was a VO2 max projected from submaximal lab tests, not a measured maximal test. That difference in reference likely explains part of why Apple's error looks smaller than in the independent studies.

Metric	Apple validation group (N = 221)
Mean error vs reference	1.4 ± 4.7 mL/kg/min
Reliability (ICC)	0.86
Median variation per person	1.2 mL/kg/min (3.4%)
Workouts over 5.75 min giving an estimate	78%

Caption: Apple, "Using Apple Watch to Estimate Cardio Fitness with VO2 max", May 2021, Table 2. Reference = submaximal projection of VO2 max.

Apple's sample also differed from the independent studies: average age 55, about 38% over 65, and a mean reference VO2 max near 30 mL/kg/min. The independent studies tested younger, fitter people. Accuracy may differ across these groups, which fits the fitness-level pattern above.

A 2022 meta-analysis by the INTERLIVE network looked across wearable brands. Devices that estimate VO2 max from exercise data had almost no average bias (−0.09 mL/kg/min) but individual limits of agreement of about ±10 mL/kg/min. Devices that use only resting data overestimated by 2.17 mL/kg/min on average, with wider limits.

Why can Apple Watch VO2 max be wrong?

Apple Watch VO2 max can be wrong when its inputs are wrong or when your heart rate does not reflect the work you are doing. Apple's white paper lists the main causes: incorrect age, sex or weight in the Health app, carrying heavy loads, soft ground like sand, pushing a stroller, and factors that raise heart rate such as dehydration, caffeine, heat and altitude.

Cause	Likely effect on the estimate
Heat, dehydration, caffeine, recent move to altitude	Lower (underestimate)
Heavy backpack, carrying a child, sand	Lower
Heart-rate-limiting medication not entered in Health	Higher (overestimate)
Wrong weight, age or sex in Health	Consistently off in either direction
Pregnancy	May be inaccurate
Conditions where heart rate does not rise normally	Higher

Caption: Apple VO2 max white paper, May 2021.

Medications matter a lot. In Apple's validation group, people taking beta blockers or calcium channel blockers had an average error of 11.8 mL/kg/min when the medication was not entered, and 1.6 mL/kg/min when it was.

How can you get a more accurate Apple Watch VO2 max?

You get a more reliable Apple Watch VO2 max by giving it more good data. Apple says accuracy improves with frequent outdoor walks, runs or hikes, higher exertion during those workouts, and wearing the watch through the day. Keep your Health Details current and record any heart-rate-limiting medication.

1. Do regular outdoor workouts. Use the Outdoor Walk, Outdoor Run or Hiking workout types.
2. Choose flat routes. Less than 5% incline or decline.
3. Push the effort. Easy strolls may not raise heart rate enough.
4. Check Health Details. Age, sex, height, weight and medications.
5. Avoid bad conditions. Heat, dehydration and heavy loads tend to lower the estimate.
6. Read the trend, not one value. A change over months is more meaningful than a single estimate.

What does low cardio fitness on Apple Watch mean?

Apple's low cardio fitness notification means your estimated VO2 max is consistently in a low range for your age and sex. For ages 20 to 59, Apple uses the lowest fifth of the FRIEND registry. For ages 60 and over, it uses 18 mL/kg/min for men and 15 for women. You need to opt in to get these notifications.

Because the watch can underestimate, check the conditions above first. A low estimate is still a useful prompt to move more. If you also get breathless or have chest discomfort with light effort, see a doctor. To see where your number falls, use the VO2 max by age chart, and for ways to raise it, read how to improve VO2 max.

How Svanu handles VO2 max

Svanu uses the VO2 max in Apple Health when your watch provides one. If there is none, it calculates an estimate and labels it as one: from your resting heart rate and age (the Uth-Sorensen ratio) when it has a resting heart rate baseline, or otherwise from age, sex, BMI and average daily steps (a non-exercise model based on Jackson et al. 1990). Based on the INTERLIVE meta-analysis above, resting-data estimates are less precise than exercise-based ones, so treat the label seriously.

VO2 max is also one of five signals in the app's biological age estimate, which shows a wider likely range when fewer signals are available. Read the recovery score guide for how fitness fits with recovery and training load, or see the Svanu home page.