Cardio Fitness: What VO₂ Max Tells You and How It Is Measured

VO₂ max is the maximum volume of oxygen your body can extract from the air and deliver to working muscles for energy. It reflects the combined performance of your lungs bringing in oxygen, your heart and blood vessels transporting it, and your muscles using it at the cellular level. Because it captures the efficiency of this entire process, VO₂ max is a strong overall indicator of cardiorespiratory fitness. The value is normalised for body mass and expressed in millilitres of oxygen per kilogram per minute (mL/kg/min).

Accuracy can be affected by:

• Incorrect age, sex, or weight entered in the app
• Lung disease, dehydration, extreme heat, moving to high altitude, caffeine intake, pregnancy
• Carrying additional load (backpack, child, weights) or running on uneven terrain, pushing a stroller, all of which increase effort beyond what the metric accounts for

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How Apple Watch Estimates It

Apple Watch Series 3 or later estimates VO₂ max using heart rate and motion sensors during outdoor walks, runs, and hikes. It also factors in your:

• Age
• Sex
• Weight
• Height
• Medications that may affect heart rate

Apple Watch supports a validated range of 14 to 65 mL/kg/min for users aged 20 and older. An initial estimate requires at least 24 hours of watch use alongside logged outdoor workouts and passive measurements. If Apple Health later determines that earlier values can be refined, you may see an option to update them under Health > Cardio Fitness.

Source: 

Track your cardio fitness levels, apple.com