Login | December 28, 2025
How reliable is the VO2 number on your smart watch?
PETE GLADDEN
Pete’s World
Published: December 29, 2025
Over the past couple of years most of us have become familiar with the term VO2 max, and that’s because it’s become a hot topic in the fitness community what with health and wellness specialists advocating it as an important predictor of one’s longevity.
Now let me deviate for just a second for those of you who still can’t wrap your head around this VO2 max thing.
VO2 max is a measure (calculated in milliliters of oxygen per kilogram of body weight) of how much oxygen a person’s body can use during a bout of maximum intensity exercise.
It’s typically measured in a lab setting (or outdoors using super portable and expensive equipment) with exercises like cycling, running and cross country skiing.
Thus, for exercise physiologists VO2 max is the gold standard for assessing cardiovascular fitness - because it’s the best indication how well the heart, lungs and muscles are working together to create energy.
Okay, so with that being said it’s no surprise then that most of today’s fitness watches provide their wearers with VO2 max calculations.
And remember, this is without the person doing a maximal bout of exercise…in a lab setting…amongst trained specialists.
This also means that watch calculations are created minus all the expensive facemarks, hoses, computers, HR monitors, power meters and O2 measuring devices that are readily available in physiology labs.
So all this begs the question: Are watch VO2 max calculations as accurate - or as close to as accurate - as the calculations one would obtain in a lab setting?
Now before we dig into the results from several studies which took up this question, let’s first go over a few reasons why sports watch calculations could differ from lab calculations.
First, those lab calculations directly and precisely measure O2 consumption with high tech equipment, whereas sport watches use algorithms to estimate one’s VO2 max based on variables like HR (heart rate), exercise history and demographics.
Then you have HR accuracy issues.
Wrist devices that measure HR tend to be far less reliable than both the ECG monitoring equipment in labs and those over-the-counter chest-strap transmitters you get with high end HR monitors.
And finally there’s always a host of outside variables which can affect a wrist watch’s HR readings, things like temp, humidity and one’s own sweat.
Okay, now to the studies.
A current study, “Investigating the accuracy of Apple Watch VO2 max measurements: A validation study,” published on May 15, 2025 in Plos One (a peer-reviewed scientific journal published by the Public Library of Science), determined that the “Apple Watch underestimated VO2 max, with a mean difference of 6.07 mL/kg/min (95% CI 3.77-8.38)” when compared to a lab tests using the same people in the same environmental conditions.
Now for competitive athletes a 6 mL/kg/min variation is a big discrepancy if they’re trying to set up reliable HR training zones.
Then by the same token, for recreational athletes this variance may not be that big of a deal.
And the researchers agreed with that latter assessment, saying, “However, further consideration of Apple Watch as an alternative to conventional VO2 max prediction from submaximal exercise is warranted, given its practical utility.”
Then there’s this study, “Validity of Estimating the Maximal Oxygen Consumption by Consumer Wearables: A Systematic Review with Meta-analysis and Expert Statement of the INTERLIVE Network,” published in the British Journal of Sports Medicine on Jan. 4, 2021.
It found that, “The exercise-based estimation seems to be optimal for measuring VO2max at the population level, yet the estimation error at the individual level is large, and, therefore, for sport/clinical purposes these methods still need improvement.”
And in another study, “Validity of Wrist-Worn Activity Trackers for Estimating VO2max and Energy Expenditure,” published in the August 2019 edition of International Journal of Environmental Research and Public Health, the researchers determined that, “Tested devices did not show valid results concerning the estimation of VO2max and EE [energy expenditure].
Hence, the current wrist-worn activity trackers are most likely not accurate enough to be used for neither purposes in sports, nor in health care applications.”
So what these studies tell me is this: If you’re a serious athlete who needs accurate VO2 max information, well, the lab’s going to be the way to go.
And conversely, if you’re a recreational athlete looking to improve your VO2 max without an expensive lab test then the wrist watch can certainly aid you in your quest.