How Accurate Is My Smartwatch's VO2 Max Estimate?

Twenty-eight adults ran a maximal treadmill test with a gas mask on, and their Apple Watches had already given them a VO2 max estimate from a week of outdoor walks and runs. The watch came in low by an average of 6.07 mL/kg/min, with a mean absolute percentage error of 13.31 percent (see [1]). For one runner, the gap between watch and lab ran anywhere from about 6 points too high to 18 points too low. That is the difference between being told you are average and being told you are fit.

The bigger picture is a bit kinder. A 2022 meta-analysis of 14 validation studies and 403 people found that devices estimating VO2 max from exercise data were nearly unbiased across the group, off by just 0.09 mL/kg/min on average, while devices working from resting data overestimated by about 2 points (see [2]). The catch sits in the spread. Even the exercise-based estimates had limits of agreement near 10 points either way, and the authors judged them fine for population research but too imprecise for individual sport or clinical decisions.

Fitness level seems to matter too. When researchers tested a Garmin Forerunner 245 against lab gas analysis in 35 endurance athletes, the watch underestimated VO2 max overall, and the error grew larger and agreement weaker in the highly trained group than in the moderately trained one (see [3]). The Apple Watch sample was also mostly very fit, and its authors suggested that may explain part of the underestimation (see [1]). So the runners who care most about the number are the ones it describes least well.

Here is how I read it as a coach. Treat the watch figure as a trend line, not a lab result. Compare it only against itself, on the same device, built from steady outdoor runs with a good heart rate signal, and look at weeks, not single days. If it climbs through a base block while your easy pace drops at the same heart rate, the training is working, whatever the absolute number says. If you want the real value, a graded lab test is still the reference. For what the number actually measures and why it matters for distance running, our guide to VO2 max walks through it.