VO₂ max is the greatest amount of oxygen your body can use during intense exercise. A smartwatch estimates it from other information; it does not directly measure the oxygen you breathe in and use. That makes the number convenient, but different from a laboratory measurement. Laboratory testing; Apple’s estimation method
The useful question is not simply whether a watch is “accurate”. It is how closely the estimate agrees with a reference test, for which people, and on which hardware and software.
What VO₂ max measures
VO₂ max describes aerobic capacity. A common unit is millilitres of oxygen per kilogram of body weight per minute, written mL/kg/min.
In an exercise laboratory, a person wears a metabolic mask while the workload on a treadmill or bike progressively increases. Breathing measurements allow oxygen use to be assessed during the test. The University of Virginia’s laboratory describes this approach as a way to measure cardiorespiratory fitness. How a laboratory test works
A wrist device takes a different route: it uses an algorithm to predict a result from signals and personal information collected without that breathing equipment.
How Apple Watch produces the estimate
Apple’s UK documentation, updated on 15 September 2026, says its estimate uses heart and motion sensors during Outdoor Walk, Outdoor Run or Hiking workouts. Indoor workouts do not count towards this estimate.
Age, sex, height, weight and medications that affect heart rate also enter the calculation. Apple says an initial estimate can require at least 24 hours of wear, several qualifying workouts and passive measurements. Apple’s documented inputs
These requirements help explain why recording exercise does not always produce a new VO₂ max value. They describe this Apple feature; another manufacturer’s eligible activities and algorithm may differ.
What accuracy studies actually show
A 2022 INTERLIVE review combined 14 validation studies. Estimates using exercise data performed better overall than estimates based on resting information. Yet substantial individual error remained, even when the average difference across a group was small. INTERLIVE review
That distinction matters: overestimates for some people and underestimates for others can partly cancel in a group average. A close average does not guarantee a close result for one wearer.
A separate 2025 University College Dublin study recruited 30 adults; 28 met its criteria for the final comparison. It compared Apple Watch estimates with a maximal treadmill test using indirect calorimetry—measurement of respiratory gases. Most participants had relatively high fitness, and the devices ran watchOS 10 or later. Study methods and participants
The laboratory result was, on average, 6.07 mL/kg/min higher than the watch estimate, with a 95% confidence interval of 3.77–8.38 for that mean difference. Individual differences varied considerably. The confidence interval describes uncertainty around the group’s mean difference, not the error range for every person. Study results
Adding 6.07 to a personal reading would not correct the watch: the finding belongs to that study population and comparison. It is also not a verdict on every later device or algorithm.
Why a changed number needs context
Apple documents another reason a displayed value can change: the Health app may offer to update earlier cardio-fitness estimates when it detects that they could be more accurate. The app then shows how those estimates changed. Apple’s data-update explanation
A number on a watch is therefore both a physiological estimate and the output of a particular calculation. Its meaning depends on the source data, the algorithm and the evidence validating that combination.
For a similar distinction between recorded information and a calculated rating, see our explanation of what wearable sleep scores measure.