Apple’s Health app offers a wide array of metrics that it can measure and track. For those focussed on aerobic fitness, tracking your VO2 Max is a key measure. If you follow various health and fitness sites and discussions, one of the most talked about topics is the accuracy of Apple’s measurement of VO2 Max. Some recent analyses, published in respected research journals shed some light on how Apple determines VO2 Max.
What is VO2 Max?
VO2 Max is a measure of how well your body can use oxygen. Your VO2 Max increases through exercise as lung capacity, heart efficiency and your circulatory system adapt and improve.
As your body gets better at using oxygen, your VO2 Max increases.
Apple uses algorithms that are based on a number of variables. Factors such as heart rate, age and weight are all used. In a document that describes how Apple uses the Apple Watch to predict VO2 Max, it says “Cardio fitness on Apple Watch is an estimation of a user’s VO2 max in ml/kg/min, made based on measuring a user’s heart rate response to physical activity.“
VO2 max is a measure of the volume of oxygen (ml) is used according to body weight (kg) each minute (min).
If you get faster running over a specific distance while maintaining a similar heart rate and lose weight your VO2 Max should increase. That’s because you are using about the same amount of oxygen as your weight decreases. But, as you get older, the algorithm may lower your score. It uses a predictive model that takes ageing into account.
It’s important to note that the Apple Watch can’t actually measure VO2 Max. It uses algorithms to make a prediction. Experts accurately measure VO2 Max by placing someone on a treadmill, exercise bike, or similar device, monitoring the oxygen inhaled and exhaled, and tracking their heart rate.
How accurate is Apple’s prediction of VO2 Max?
The most accurate way to measure VO2 Max requires direct measurement. But there are ways of determining VO2 Max by subjecting people to sub-maximal loads. It’s even possible to predict VO2 Max through comparing resting heart rates with maximum heart rates.
There’s some useful research you can check out if you’re interested in other VO2 measurement methods.
But where does Apple’s V02 Max measurement fit in when it comes to accuracy?
Apple changed how it calculates VO2 max in iOS 26 and applied its new algorithm retroactively on historical data. That was an optional change. But if you applied it, as I did, there’s no going back. Scanning reactions to the change on Reddit and other forums suggests that most people’s predicted VO2 Max increased when Apple changed its algorithm. In my case, the opposite was true with my score dropping by about 1.5%.
Researchers from the School of Public Health, Physiotherapy and Sports Science at University College in Dublin looked into the accuracy of Apple’s VO2 Max measurement. Their research predates Apple’s recent algorithm change but still makes for interesting reading.
They concluded that “Overall, Apple Watch underestimated VO2 max”. That aligns with Apple’s recent change that resulted, for most people, in their VO2 Max increasing as a result of the updated algorithm.
They also noted that while the Apple Watch was not a perfect tool for measuring VO2 Max that its availability (and one assumes the availability of other health-focused wearables) and utility was a reasonable compromise over perfect accuracy.
In other words, good enough overcomes the need for perfect.
Anthony is the founder of Australian Apple News. He is a long-time Apple user and former editor of Australian Macworld. He has contributed to many technology magazines and newspapers as well as appearing regularly on radio and occasionally on TV.