How Apple Watch Calculates VO2 Max (Cardio Fitness)

Apple calls it Cardio Fitness rather than VO2 max, and it is the one number where Apple publishes more rather than less. It is also the only wearable VO2 max estimate that independent researchers have put through proper validation against laboratory testing, which means we can say how wrong it is instead of guessing.

The short version: it underestimates, and the error on any individual is large.

How the estimate is produced

Apple estimates Cardio Fitness from outdoor walks, runs and hikes, using the relationship between your pace and your heart rate, with GPS supplying the pace. That is the same basic logic every brand uses, including Garmin's Firstbeat engine: at a given speed, a fitter person's heart rate is lower.

The notable difference is that Apple will produce an estimate from a brisk walk. Most brands need a run above roughly 70% of your maximum heart rate before they will commit to a number. Apple's willingness to work from lower-intensity data is why most Apple Watch owners have a Cardio Fitness figure at all, and it is plausibly part of why the estimate is conservative.

What the validation studies found

Two things make this section unusual. First, the studies exist. Second, the results are not flattering.

A 2025 validation study in PLOS One had participants wear an Apple Watch for five to ten days to generate an estimate, then measured their real VO2 max with indirect calorimetry on a maximal treadmill test. It reported a mean absolute error of 6.92 ml/kg/min and a mean absolute percentage error of 13.31%, with Apple Watch underestimating overall.

Follow-up work on the Series 10 reached the same conclusion: individual-level agreement is poor, the limits of agreement are wide, and the estimates are not suitable for clinical use on one person. The researchers' qualified positive is that the numbers may still be useful at population level, for research cohorts rather than for you.

To put 6.92 ml/kg/min in perspective: a hard training block might genuinely move your VO2 max by 2 or 3.

Estimate yours from a race result or a field test. If the two disagree by five or more points, the field test is more likely to be right.

How to use a number with that much error

As a trend, not a value. Systematic underestimation matters less if it is consistent. A figure that is 7 points low every month still shows you the direction you are moving.

Keep the inputs constant. Same route type, same terrain, same conditions. Apple's estimate comes from outdoor activity, so a month of treadmill work or indoor cycling will leave it stale rather than falling.

Do not compare it to a friend's Garmin. Different algorithms, different inputs, different biases. Cross-brand comparison of VO2 max estimates is meaningless.

Remember what sits underneath it. Every pace-versus-heart-rate model needs a maximum heart rate, and Apple never publishes what it uses for yours. That is one more unknown in the chain.

FAQ

How does Apple Watch calculate VO2 max? From outdoor walks, runs and hikes, by modelling your heart rate against your GPS pace. Apple labels the result Cardio Fitness.

Is Apple Watch VO2 max accurate? Validation work found a mean absolute error of about 6.92 ml/kg/min and a mean absolute percentage error of 13.31%, with a tendency to underestimate. Poor agreement for an individual, usable as a trend.

Why is my Apple Watch Cardio Fitness lower than my Garmin VO2 max? Because Apple underestimates relative to lab testing and the two use different models and inputs. Neither number is the truth.

Why is my Cardio Fitness not updating? Apple needs outdoor GPS activity. Treadmill running, indoor cycling and gym work do not generate an estimate.

Can Apple Watch replace a lab VO2 max test? No. The validation literature is explicit that individual estimates are not accurate enough for clinical use.

Other brands

• Garmin uses Firstbeat and lands within 3 to 5 ml/kg/min with a chest strap.
• COROS uses a rolling six-week window of outdoor runs.
• Polar will estimate it without exercise at all.
• Fitbit shows a range rather than a number if you never run with GPS.

Sources

• Apple Support: Track your cardio fitness levels
• PLOS One: Investigating the accuracy of Apple Watch VO2 max measurements
• Accuracy of VO2 max estimates from Apple Watch Series 10
• How Apple Watch calculates your max heart rate

Cover photo by Fastily, CC BY-SA 4.0, via Wikimedia Commons.