VO2 max — the maximum rate at which your body can use oxygen during exercise — is one of the most well-established measures of cardiovascular fitness in exercise science. It’s also one of the most misunderstood numbers on a home cardio machine or fitness watch, because the figure displayed on the console isn’t the same thing as a laboratory-measured VO2 max, even though it’s presented the same way.
What VO2 max actually measures
VO2 max is expressed in millilitres of oxygen consumed per kilogram of body weight per minute (mL/kg/min), and reflects the combined capacity of your heart, lungs and muscles to take in, transport and use oxygen during maximal effort. A true VO2 max test is a laboratory or clinical procedure: it involves exercising to genuine exhaustion, usually on a treadmill or bike, while breathing through a mask connected to a gas analyser that directly measures oxygen consumption and carbon dioxide output throughout the test.
How a treadmill, bike or watch actually gets to a number
Home equipment and wearables don’t do any of that direct gas measurement. Instead, they estimate VO2 max using indirect methods — typically combining heart rate data (at rest and during exercise) with factors like age, sex, weight, and sometimes pace or power output, run through a predictive algorithm built from population-level data. This is a fundamentally different approach from direct measurement, and it’s the reason the number on a console or watch face should be read as an estimate with a real margin of error, not a precise clinical figure.
What recent validation research actually found
A 2025 validation study assessing Apple Watch VO2 max estimates against gold-standard indirect calorimetry found the watch substantially underestimated VO2 max on average, by around 6 mL/kg/min, with a mean absolute percentage error of roughly 13%. The same study found the size and direction of the error varied by fitness level — other research cited alongside it suggests a pattern where devices tend to underestimate VO2 max for fitter individuals and overestimate it for less fit individuals, meaning the error isn’t simply a fixed, predictable offset you can mentally correct for. This kind of algorithm-based estimation is broadly similar in principle across consumer wearables and cardio equipment, even though the exact methodology differs by brand and device.
Why this still doesn’t make the number useless
An estimate with a roughly 13% average error sounds like it should be dismissed, but that’s not quite the right conclusion for how most people actually use it. Tracked consistently over time, on the same device, using the same estimation method, a rising or falling trend in estimated VO2 max is still meaningful information about whether your cardiovascular fitness is improving — even if the absolute number itself isn’t clinically precise. Where the estimate becomes genuinely unreliable is in treating a single reading as an exact, comparable-to-anyone-else figure, or in comparing numbers across different brands of equipment or watch, since each uses its own algorithm and reference population.
What this means for choosing or using home cardio equipment
If a VO2 max estimate feature is a factor in choosing between cardio machines, it’s worth treating it as a nice-to-have trend indicator rather than a headline reason to prefer one machine over another — the underlying hardware quality, comfort, programming options and durability of the equipment itself matter considerably more for actual training outcomes than which one happens to display an estimated VO2 max figure. For anyone who genuinely needs an accurate, clinically meaningful VO2 max figure — for a specific health condition, a research context, or serious competitive training — a proper laboratory or clinical cardiopulmonary exercise test remains the only way to get one.
The bottom line
VO2 max estimates on home cardio equipment and wearables are built on indirect, algorithm-based methods, and validation research shows a real average error in the region of 13%, with accuracy varying by individual fitness level. Treat the number as a rough, device-specific trend indicator you can track consistently over time, not as an accurate substitute for a real laboratory test.
Sources
- Validation study, “Investigating the accuracy of Apple Watch VO2 max measurements,” 2025 — pmc.ncbi.nlm.nih.gov