How Accurate Is Your Watch's VO₂ Max Estimate?

A watch’s VO₂ max is an estimate, not a measurement. It’s usually right on average across many people, but for one person it can be off by 5 mL/kg/min or more, the difference between two fitness categories. Its strength is the trend: the same watch, used the same way, can show whether your fitness is rising or falling.

This article is general information, not medical advice.

How Watches Estimate It

A true VO₂ max test measures the oxygen you breathe while you exercise to exhaustion; see What Is VO₂ Max? A watch can’t measure oxygen. Instead it relies on a simple relationship: at the same pace or power, a fitter heart beats more slowly. During outdoor walks, runs, or rides, the watch pairs your heart rate with your speed from GPS (and sometimes your power), then extrapolates to what your maximum would be, using your age, sex, weight, and its maker’s equations. Some devices also use resting measures.

What the Studies Found

• Across devices. A 2022 meta-analysis by Molina-Garcia and colleagues, for the INTERLIVE network, found that wearables estimating VO₂ max from exercise had almost no average bias, but individual estimates could be off by about 10 mL/kg/min either way. Devices using only resting information overestimated by about 2 mL/kg/min on average, with even wider errors.
• Apple Watch. In a 2025 validation study by Lambe and colleagues, Apple Watch underestimated VO₂ max measured on a treadmill by about 6 mL/kg/min on average, with an average error of about 13%.

So for a population, watch estimates are useful; for you alone, a single value is a rough guess.

Getting a Useful Trend

1. Do the activity the watch learns from. Most need outdoor walks or runs on fairly flat ground, with GPS, at a brisk effort, often for 20 minutes or more.
2. Keep your profile current. Weight, age, and sex enter the equations; an old weight skews the estimate.
3. Wear it snugly. Wrist heart rate errors during exercise feed straight into the estimate.
4. Look over months. A change of 1 or 2 points in a week is noise; a steady rise over three months is real.
5. Expect a new watch to differ. Different models and brands use different equations. Start a new trend rather than joining the two.

For a number you can rely on, a cardiopulmonary exercise test at a sports science lab or clinic measures VO₂ max directly.

Following It in Kaizora

Kaizora keeps a watch’s Estimated VO₂ Max and a lab test’s VO₂ Max as separate metrics, so an estimate never averages into a measurement. Apple Health’s export brings in Apple Watch’s estimates as Estimated VO₂ Max, and a value Apple Health marks as from a maximal exercise test as VO₂ Max. Each value records its device, so the chart can be broken down by device when you change watches. On Fitness, each metric’s page shows your personal best.

The Short Version

• Watches estimate VO₂ max from heart rate and pace, not by measuring oxygen.
• They’re right on average, but individuals can be off by 5 to 10 mL/kg/min.
• Follow one watch’s trend over months, and measure in a lab for an exact number.

Sources

• Molina-Garcia P et al., “Validity of estimating the maximal oxygen consumption by consumer wearables: a systematic review with meta-analysis and expert statement of the INTERLIVE network,” Sports Medicine 52, no. 7 (2022): 1577–1597. doi:10.1007/s40279-021-01639-y
• Lambe RF et al., “Investigating the accuracy of Apple Watch VO₂ max measurements: a validation study,” PLOS One 20, no. 5 (2025): e0323741. doi:10.1371/journal.pone.0323741