Is the Apple Watch's VO2 max estimate actually accurate?
Two independent validation studies tested Apple Watch's wrist-based VO2 max against real lab tests, and the gap was biggest for exactly the people most likely to trust the number.
By Zain
The claim: your Apple Watch knows your VO2 max
Apple Watch has quietly turned VO2 max, a number that used to require a lab, a mask and a maximal treadmill test, into something that appears on your wrist after a few runs. Apple markets it as a meaningful fitness and even longevity metric, and plenty of people now treat the number as gospel: a 42 means something specific and comparable to a 42 from a real lab test.
Two independent validation studies, published in 2024 and 2025, put that assumption to the test against the gold standard: a maximal exercise test with a metabolic cart measuring actual oxygen uptake.
What the validation studies found
The larger of the two, a 2025 study of 28 adults in PLoS One, compared Apple Watch estimates against indirect calorimetry on a treadmill. The watch underestimated lab-measured VO2 max by an average of 6.07 mL/kg/min, a mean error of 13.31%. The range was wide: the study's limits of agreement ran from -6.11 to as much as 18.26 mL/kg/min off the true value, meaning an individual reading could be well off even when the average error looks modest. The authors are direct about the implication: "Apple Watch may provide a suitable estimation of aerobic capacity for general fitness monitoring," but "our findings suggest that estimates are not sufficiently accurate to inform clinical decision-making."
A smaller 2024 study of 19 adults in JMIR Biomedical Engineering, using an Apple Watch Series 7 and a cycle ergometer test, found a similar pattern: the watch underestimated VO2 max by 4.51 mL/kg/min on average, a 15.79% mean error. The more striking finding was how unevenly that error landed. People with "Excellent" fitness levels had the worst accuracy, undershooting by 12.00 mL/kg/min on average with a 21.47% error, while people with lower fitness levels saw smaller, sometimes opposite, errors. The statistical reliability score (ICC) for the whole sample was 0.47, which the authors themselves describe as poor. Their blunt framing: "there is a lack of scientific validation for VO2max estimation outside the laboratory setting."
“Our findings suggest that estimates are not sufficiently accurate to inform clinical decision-making.”
Why the error gets worse for fitter people
Both studies point to the same likely cause: Apple's algorithm leans on heart rate and pace during ordinary walks and runs, which is a reasonable proxy at moderate effort but a poor one for people who rarely push hard enough during daily activity to generate the heart-rate-to-pace relationship a true VO2 max test captures. A dedicated runner who never does a flat-out effort on a normal run can end up with a watch estimate that undersells real fitness by more than a lab test ever would. That matters for anyone using the number to judge whether training is working, and it's a big reason the number can swing noticeably after you change routes, weather or how hard a given run actually felt.
What this is still useful for, and what it isn't
Neither study calls the feature useless. Tracked over months, a rising or falling trend on your own wrist is a reasonable signal of your own fitness direction, since the same algorithm's bias tends to apply consistently to the same person. What it isn't is a number you should treat as clinically precise or directly comparable to someone else's lab result, and it's not the number to lean on for the specific training zones or detailed age norms we cover in our explainer on what actually counts as a good VO2 max. If you genuinely need a precise figure, for a medical reason or serious training plan, a supervised treadmill or bike test with gas analysis is still the only way to get one.
“There is a lack of scientific validation for VO2max estimation outside the laboratory setting.”
Our take
Apple Watch's VO2 max estimate isn't fake, but it isn't a lab result either. Across two independent studies, it ran roughly 13 to 16% off on average, with the biggest misses landing on the fittest people, exactly the group most likely to care about precision. Use the trend line, not the single number, and don't let a watch score talk you into or out of a real fitness assessment.
This is general information, not medical advice. Talk to a clinician before changing anything, especially if you are pregnant, nursing, or managing a condition.
Frequently asked questions
How accurate is the Apple Watch VO2 max estimate?
Moderately, not precisely. Two validation studies against lab metabolic-cart tests found average errors of 13.31% and 15.79%, with the watch typically underestimating true VO2 max. Individual readings varied even more than the averages suggest, so treat any single number as a rough estimate rather than a lab-grade result.
Does Apple Watch overestimate or underestimate VO2 max?
Both validation studies found it mostly underestimates, by an average of 6.07 mL/kg/min in one study and 4.51 mL/kg/min in the other. The underestimate was largest for the fittest participants, who saw average errors above 20%, while less fit participants saw smaller, more mixed errors.
Can I use my Apple Watch VO2 max score to train?
You can use the trend over weeks or months as a rough signal of fitness direction, since the same bias tends to apply consistently to one person over time. For setting precise training zones or judging a single number against population norms, the margin of error is too wide to rely on.
What is a good VO2 max on Apple Watch?
The watch's own number should be read as approximate rather than exact, since validation studies found average errors of 13 to 16% against lab testing. For a sense of what counts as a strong score by age and sex using proper reference data, a lab-based or clinically validated chart is more reliable than the wrist estimate alone.
How does the Apple Watch calculate VO2 max?
It estimates VO2 max from heart rate and pace data gathered during ordinary outdoor walks and runs, using an algorithm rather than measured oxygen uptake. That approach works reasonably at moderate effort but loses accuracy for people who rarely reach a hard enough effort during normal activity to match what a true maximal test captures.
Is a smartwatch VO2 max as good as a lab test?
No. A lab test measures oxygen uptake directly during a maximal effort using a metabolic cart, which validation studies still treat as the reference standard. Smartwatch estimates, including Apple Watch's, ran 13 to 16% off that standard on average in independent studies, so they're a convenient approximation, not a replacement.