Apple Watch estimates your VO2 max automatically during outdoor walks, runs, and hikes, then displays it as “Cardio Fitness” in the Health app. You don’t need to run a special test. But your watch does need the right setup and the right kind of workouts before it will generate a reading.
What You Need Before It Works
You need an Apple Watch Series 3 or later, and you must be at least 20 years old. The watch won’t generate a VO2 max estimate for anyone under that age threshold.
Your Health profile needs to be filled out in the Health app on your iPhone, including your age, sex, weight, and height. The watch factors all of these into its calculation. If you take any medications that affect your heart rate (beta blockers, for example), you can note that in the Health app too, and the algorithm will adjust accordingly.
After your profile is set up, plan on wearing your Apple Watch for at least 24 hours before expecting any data. The watch needs baseline measurements of your resting heart rate and general activity patterns before it can interpret your exercise performance.
Which Workouts Actually Count
Only three workout types trigger a VO2 max estimate: Outdoor Walk, Outdoor Run, and Hiking. These must be started through the Workout app on your watch. Indoor treadmill runs, cycling, swimming, and other activities won’t produce a reading.
The terrain matters too. Your route needs to be on relatively flat ground, with less than a 5% incline or decline. Steep hill workouts throw off the algorithm because the relationship between heart rate and pace changes dramatically on hills.
During the workout, two things need to happen. First, your watch must get a strong GPS signal and a clean heart rate reading. A loose watch band or poor satellite coverage can prevent a measurement. Second, your heart rate needs to rise by roughly 30% above your resting value. A very easy stroll that barely elevates your pulse won’t give the watch enough data to work with.
You also won’t get a number after just one workout. Apple requires multiple qualifying sessions before generating an initial estimate, though the exact number varies from person to person. Most people see their first reading after a handful of outdoor workouts spread over a few days.
How the Watch Calculates Your Score
The Apple Watch doesn’t measure VO2 max directly. True VO2 max testing requires breathing into a mask while exercising to exhaustion in a lab. Instead, the watch uses a submaximal estimation method: it tracks how your heart rate responds to a given pace and uses that relationship to predict what your maximum oxygen uptake would be.
The logic is straightforward. If your heart rate stays relatively low while you walk or run at a brisk pace, your cardiovascular system is efficient, and your estimated VO2 max will be higher. If your heart rate spikes quickly at the same pace, the estimate will be lower. The watch combines this heart rate-to-pace ratio with your profile data (age, sex, weight, height) to produce a number measured in mL/kg/min.
Your watch also takes passive measurements throughout the day, using your resting heart rate and walking pace to refine its estimate over time. This means your Cardio Fitness score can update even on days you don’t do a formal workout.
Where to Find Your Results
Open the Health app on your iPhone and tap the Browse tab. Go to Heart, then tap Cardio Fitness. You’ll see your most recent VO2 max estimate along with a trend graph showing how your fitness has changed over weeks and months.
Apple classifies your score into one of four levels based on your age and sex: Low, Below Average, Above Average, or High. These ranges are drawn from population-level fitness data, so “Above Average” means you’re fitter than most people of the same age and sex.
Turning On Low Fitness Alerts
Apple Watch can notify you if your cardio fitness drops into the Low range, which research has linked to higher risks of heart disease and other chronic conditions. To enable this, open the Apple Watch app on your iPhone, tap My Watch, then tap Heart, and select “Set Up Cardio Fitness Notifications in Health.” Follow the prompts to turn on alerts. If your VO2 max dips below the threshold for your demographic, you’ll get a notification.
How Accurate Is It?
The Apple Watch provides an estimate, not a lab-grade measurement. A validation study published in PLOS One found that the watch generates its scores by analyzing heart rate response during outdoor exercise on flat terrain, relying on GPS pace and optical heart rate data. This approach is inherently less precise than a metabolic cart test where you breathe into a mask while running on a treadmill.
In practice, wrist-based estimates tend to be most reliable for tracking trends over time. If your Cardio Fitness score climbs steadily over three months of training, that’s a meaningful signal that your cardiovascular fitness is improving. The absolute number might be off by a few points compared to a lab test, but the direction of change is useful information. Where the watch is less reliable is in single-point accuracy for highly trained athletes or people with irregular heart rhythms, since both can confuse the algorithm.
Tips for More Reliable Readings
Wear your watch snugly about one finger-width above your wrist bone. A loose band lets light leak under the sensor and degrades heart rate accuracy. Make sure Location Services are enabled for the Workout app so GPS tracking works properly.
Choose flat routes when you want a clean VO2 max update. Hilly terrain beyond a 5% grade won’t generate an estimate, and even moderate hills can add noise to the calculation. Consistent outdoor workouts of at least 20 minutes give the watch the most data to work with.
Keep your Health profile current. If your weight changes by more than a few pounds, update it in the Health app. The algorithm uses your weight in its calculation, so outdated numbers will skew results. Same goes for any new heart rate medications you start or stop taking.