Apple Watch VO2 Max Accuracy

Apple Watch VO2 Max estimates are generally within 1-2 ml/kg/min of lab results for most users. That is good enough to track trends reliably, which is what matters for health. You do not need laboratory precision to know whether your cardio fitness is improving, stable, or declining. The trend over weeks and months is far more useful than any single reading.

How Apple Watch Calculates VO2 Max

Apple Watch uses heart rate and motion sensors during outdoor walks, runs, and hikes to estimate your VO2 Max. The algorithm combines your heart rate response with pace, elevation, and personal data (age, sex, weight, height) to produce a submaximal prediction. It does not measure your peak oxygen uptake directly. Instead, it estimates what your maximum would be based on how your heart responds to moderate exertion.

The validated range is 14-65 ml/kg/min for users aged 20 and older. The estimate requires flat or moderate terrain (under 5% grade), adequate GPS signal, and sufficient exertion during the workout. A casual stroll may not provide enough data for a reliable reading.

What the Research Shows

A 2023 validation study from the Technical University of Darmstadt (published in JMIR mHealth and uHealth) found that the Apple Watch Series 7 showed reasonable agreement with laboratory cardiopulmonary exercise testing. The watch tended to slightly overestimate VO2 Max in lower-fitness individuals and slightly underestimate it in higher-fitness individuals. This compression toward the middle is a common pattern across all consumer wearables, not unique to Apple Watch.

For most users in the typical fitness range, the error is small enough that it does not affect the practical value of the measurement. Your age-group percentile ranking based on the Apple Watch estimate will usually be correct or very close.

Why Your Reading Might Seem Wrong

Several common scenarios produce inaccurate VO2 Max readings on Apple Watch:

Cool-down walks after intense exercise. If you finish a HIIT session and immediately start a walking cool-down, your heart rate is still elevated from the prior workout. Apple Watch sees a high heart rate at a low walking pace and estimates a low VO2 Max. This is one of the most common causes of unexpectedly low readings. For specific reasons your reading might drop, see Why Did My Apple Watch VO2 Max Drop?

Indoor workouts only. Apple Watch does not estimate VO2 Max from treadmill runs, cycling, swimming, or gym sessions. If you rarely exercise outdoors, your reading may be stale or based on limited data. Learn more about how often Apple Watch updates your VO2 Max and what triggers a new reading.

Loose watch fit. Optical heart rate sensors need consistent skin contact. A loose band introduces noise into the heart rate data, which can skew the VO2 Max calculation in either direction.

Heart rate medications. Beta blockers and other medications that limit heart rate response will produce artificially low VO2 Max estimates because the algorithm interprets a suppressed heart rate differently than a naturally low one.

Apple Watch vs Garmin

Both Apple Watch and Garmin use submaximal heart rate estimation to predict VO2 Max. Neither is definitively more accurate than the other. Garmin tends to read slightly higher for some users, partly because it uses Firstbeat Analytics algorithms that incorporate additional variables like breathing rate estimation. Apple’s algorithm is more conservative.

The important thing is consistency within one device over time. Comparing absolute numbers between an Apple Watch and a Garmin is less meaningful than tracking your trend on whichever device you wear daily. Luen reads your VO2 Max from Apple Health and shows your trend alongside the daily habits that influence it.