Apple Watch VO2 max estimates are off by about 13% on average compared to gold-standard lab testing, based on validation studies that put the watch head-to-head against clinical metabolic analyzers. That’s enough error to be useful for tracking trends over time, but not precise enough to treat any single reading as your true number.
What the Validation Studies Show
A 2025 study published in PLOS ONE compared Apple Watch VO2 max readings directly against indirect calorimetry, the gold-standard method where you wear a mask that analyzes every breath during a maximal treadmill test. The mean absolute percentage error came in at 13.3%, with an average absolute error of about 7 mL/kg/min. To put that in practical terms, if your true VO2 max is 50 mL/kg/min, the Apple Watch might show you anywhere from roughly 43 to 57.
More importantly, the Apple Watch consistently underestimated VO2 max in people with high fitness levels. The average underestimation was about 6 mL/kg/min. So if you’re a serious runner or cyclist, your real cardiorespiratory fitness is likely better than what your watch reports. The range of disagreement between the two methods was wide, spanning from about -6 to +18 mL/kg/min, meaning individual readings can be significantly off in either direction even when the group average looks reasonable.
Your Fitness Level Changes the Accuracy
The Apple Watch doesn’t miss by the same amount for everyone. Research published in JMIR Biomedical Engineering found a clear pattern: the watch tends to overestimate VO2 max for people with lower fitness levels and underestimate it for people with higher fitness levels. If you’re just getting into exercise, your number might look a bit more flattering than reality. If you’re already fit, the watch is likely selling you short.
This makes sense given how the algorithm works. Apple hasn’t published its exact formula, but technical discussions with Apple revealed the watch uses three main inputs: your resting heart rate, your heart rate during exercise, and GPS-derived pace from outdoor workouts. It’s essentially inferring your oxygen capacity from how hard your heart works at a given speed. That inference gets less reliable at the extremes of the fitness spectrum, where heart rate response to exercise doesn’t follow average population curves as neatly.
How the Watch Generates Your Estimate
The Apple Watch doesn’t measure VO2 max directly. A true measurement requires analyzing the oxygen and carbon dioxide in your breath during all-out exertion. Instead, the watch estimates it by combining heart rate data with movement speed during outdoor walks or runs. It needs at least 24 hours of continuous wear time before it will generate an initial estimate, and Apple states that logging more outdoor workouts improves accuracy over time as the algorithm learns your individual patterns.
Because the algorithm relies heavily on heart rate, anything that affects your heart rate independently of fitness will throw off the reading. Caffeine, alcohol, poor sleep, stress, dehydration, and certain medications (particularly beta-blockers or stimulants) can all shift heart rate enough to distort the estimate. Validation studies specifically excluded participants taking medications or with cardiovascular conditions, which means the accuracy numbers we have may not apply if either of those describes you. The watch also needs a solid GPS signal to accurately measure your pace, so treadmill runs and indoor workouts won’t update your VO2 max reading.
Newer Models Don’t Appear More Accurate
If you’ve been eyeing an upgrade hoping for a better VO2 max reading, the data suggests it won’t help. The PLOS ONE validation study found no trend in accuracy based on Apple Watch model. Whether participants wore an older Series model or a newer one, the degree of error was essentially the same. The VO2 max estimation appears to be limited more by the algorithm’s approach (inferring oxygen consumption from heart rate and pace) than by sensor hardware. Better optical heart rate sensors may reduce noise in the raw data, but the fundamental challenge of estimating a respiratory measurement from wrist-based data remains.
What the Number Is Actually Good For
A 13% error sounds discouraging if you want to know your exact VO2 max. But exact numbers aren’t really the point for most people. The Apple Watch is better suited as a trend tracker than a precision instrument. If your reading climbs from 38 to 42 over several months of consistent training, your fitness almost certainly improved, even if neither number is perfectly calibrated. The direction of change is more reliable than any single data point.
Where the watch falls short is in comparisons. Using your Apple Watch number to see how you stack up against published VO2 max norms for your age and sex is unreliable when the reading could be off by 7 mL/kg/min in either direction. That gap is large enough to move you across entire fitness categories. Similarly, comparing your Apple Watch number to a friend’s Garmin or Fitbit number is meaningless, since each brand uses a different estimation algorithm with its own biases.
For a rough, free, no-lab-required snapshot of your cardiovascular fitness that you can watch over time, the Apple Watch is a reasonable tool. For a number you can trust at face value, particularly if you’re very fit or managing a health condition, lab testing remains the only reliable option.