VO2 max

What it is

VO2 max is the highest rate at which your body can take in and use oxygen during maximal exercise, usually expressed in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min). It’s measured properly in a lab, with a mask analysing your breath during a test to exhaustion.

Why it matters for training decisions

VO2 max sets the ceiling on your aerobic engine, but it isn’t the whole story. How much of that ceiling you can sustain (see lactate threshold) and how efficiently you move both matter for race results. Two athletes with the same VO2 max can finish a long way apart.

For most athletes, the trend over months is more useful than the number itself. Treat a sudden jump or drop with suspicion until the next few estimates confirm it.

How wearables measure it, and the limits

Watches estimate VO2 max from how your heart rate responds to a known effort, not by measuring oxygen.

• Apple Watch estimates it during Outdoor Walk, Outdoor Run and Hiking workouts, using heart rate, motion and GPS plus your age, sex, height, weight and some heart rate medications. Apple says estimates need relatively flat ground and a clear rise in heart rate. Indoor workouts don’t count.
• Garmin estimates it from heart rate and running pace, or cycling power.

Hills, poor GPS or a weak heart rate signal can stop an estimate or skew it. In a 2025 study of 30 adults, Apple Watch underestimated lab-measured VO2 max on average, with wide variation between people. Use your watch’s estimate as a trend, not a lab result.

How P247 uses it

P247 reads the VO2 max estimate stored in Apple Health and treats it as an estimate, not a measured value.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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