How Garmin really estimates your fitness
When your Garmin shows VO2max, it isn’t measuring oxygen directly – it’s calculating from the data you provide. For cyclists, that means you need:
- a Garmin Edge computer or a compatible Garmin watch
- a heart-rate strap or sensor
- a power meter on your bike
The device compares how much power you can produce at a given heart rate and uses an algorithm to predict your aerobic capacity.
Apple Watches take a different approach. They cannot give you a cycling-specific VO2 max. Instead, they only generate a VO2 max estimate if you record outdoor walks, runs or hikes of at least 20 minutes with GPS and heart rate data. The question is, how accurate are Garmin and Apple at estimating your VO2 max?
What the science says so far
Independent tests of these wearable estimates have been mixed. A 2022 systematic review found that while most devices track group averages fairly well, the errors for individuals can be large, sometimes off by more than 10%. That’s good enough to spot broad fitness trends, but not always accurate if you want to know your true VO2 max in lab terms.
But 2022 is kind of a long time ago in terms of wearable tech. What is newer research showing?
The New Garmin study isn’t flawless
In 2025, researchers tested the Garmin Forerunner 245 against laboratory measurements in 35 endurance athletes. Each athlete did two outdoor runs of at least 15 minutes while wearing the watch, and their true VO2 max was also measured in the lab with the gold standard: gas exchange equipment.
- For moderately trained athletes (roughly VO2 max ≤ 60 ml/kg/min), the watch was surprisingly close, off by only about 2–3%, a margin most cyclists would consider acceptable.
- For highly trained athletes, the picture changed: the watch underestimated VO2 max by about 6 ml/kg/min, an error of around 10%.
Statistically, the agreement between watch and lab was only “moderate.” And the sample was small, just 35 athletes, so the results should be read with some caution. Still, the pattern was consistent across both test runs.
Apple vs Garmin: same trend, different accuracy
A year earlier, a study of the Apple Watch Series 7 found similar results in principle but worse in practice. Apple’s estimates were off by about 15% on average, more than double the typical error in Garmin’s case. The Apple Watch did fine in moderately fit users but showed bigger underestimations as fitness rose.
So, while both brands underestimate highly trained athletes, Garmin is clearly more accurate overall. For the average rider, Garmin’s numbers are close enough to be useful, while Apple’s tend to drift much further.
Even with the newest models, Series 9 and Ultra 2, the issue hasn’t gone away. A 2025 study found the same pattern: underestimation by about 6 ml/kg/min and errors above 10%.
What does this mean for you?
For the everyday cyclist: your Garmin’s VO2 max estimate is good enough to track your fitness journey. If the number is rising over weeks and months, that reflects real improvement, even if the absolute figure isn’t perfect.
For ambitious amateurs: if your VO2 max is at or above 60 ml/kg/min, don’t obsess over the number. Expect your Garmin to sell you short by a few points. Use it for trends, not absolutes, and if you really want the truth, a lab test is the only option.
