How Accurate Is Your Watch's VO2 Max?

Your watch shows a VO2 max number, it ticks up a point after a good month of training, and it feels like a lab result on your wrist. It isn’t. It’s an educated guess. The short answer to “how accurate is your Garmin or Apple Watch VO2 max” is: accurate enough to track your own trend, not accurate enough to treat as a precise, comparable score. It tends to read low, especially if you’re fit. This is educational information, not medical advice. If you want a number you can reason about, it helps to understand where the watch’s estimate comes from, which our VO2 max calculator mirrors using the same field-test logic.

Garlic, food, flavoring — illustrating How Accurate Is Your Watch's VO2 Max?

Where the number comes from

No consumer watch measures oxygen. A true VO2 max is measured in a lab with a mask that analyzes the oxygen and carbon dioxide in your breath during a maximal test. That’s the gold standard, called indirect calorimetry or cardiopulmonary exercise testing (CPET).

A watch has none of that. Instead it runs an algorithm (on most Garmin devices licensed from Firstbeat) that looks at the relationship between your pace (how hard you’re working) and your heart rate (how hard your body is responding) during outdoor runs, then extrapolates what your maximum oxygen uptake would be. Garmin and Apple use different proprietary models, but both lean on that same pace-to-heart-rate relationship. That design has consequences: the estimate needs GPS-based pace and a good heart-rate signal, it’s built around running, and it inherits any error in your programmed maximum heart rate.

Wristband VO2 max is an estimate from heart rate and pace during running, not a measured gas exchange value.

What the validation studies found

Researchers have put these estimates against lab CPET. The pattern is consistent: watches usually underestimate, and the group-average error is moderate while the individual error can be large.

Device (study)	Sample	Direction	Mean error	MAPE
Apple Watch (Series 5–9, Ultra 2, SE2)	28 adults	Underestimates	−6.07 mL/kg/min	13.3%
Apple Watch Series 7	19 adults	Underestimates	−4.51 mL/kg/min	15.8%
Garmin Forerunner 245	35 endurance athletes	Underestimates	≈ −4 to −4.7 mL/kg/min	6.7–7.9%

MAPE is “mean absolute percentage error”: the average size of the miss, ignoring direction. A few things stand out. In the 2025 Apple Watch validation study, the watch underestimated VO2 max by about 6 mL/kg/min on average, but the Bland-Altman limits of agreement ran from roughly −6 to +18 mL/kg/min. That spread is the real story: for any one person the watch could be close, or it could be off by a large margin in either direction. The Garmin Forerunner 245 looked tighter overall (MAPE under 8%), but that was in trained runners feeding it clean running data.

Tomato, onion, pepper — illustrating How Accurate Is Your Watch's VO2 Max?

Why it reads low when you’re fit, and low when you’re not trying

The most reliable finding across studies is that accuracy gets worse as fitness goes up. In the Forerunner 245 study, moderately trained runners saw a MAPE around 3–4%, while highly trained runners saw 9–10%. The underestimation was larger in the fitter group. The Apple Watch Series 7 study showed the same slope: it slightly overestimated in low-fitness participants but underestimated the excellent-fitness group by around 12 mL/kg/min.

The intuition is simple. These algorithms are calibrated on large, mostly average-fitness populations, so they regress toward the middle. An elite aerobic engine sits at the tail the model rarely sees, so it gets pulled down toward the crowd.

The flip side matters for everyday users too. The estimate assumes your runs probe your cardiovascular ceiling. If you only ever jog easy, walk-run, hit hills that spike heart rate, wear a loose optical sensor, or have a wrong max-HR setting, the watch has bad inputs and returns a bad number. Garbage in, garbage out.

So is it useless? No: use it as a trend

As an absolute score to compare yourself against a friend or a population chart, watch VO2 max is shaky: the individual error is too wide. As a relative signal for you, tracked over weeks and months under consistent conditions, it’s useful. The measurement error is fairly stable for a given person on a given device, so if your number climbs after a training block, that upward movement is meaningful even if the raw value is a few points off.

To get the most trustworthy trend:

Feed it real running data. Outdoor GPS runs with sustained, moderately hard efforts give the algorithm what it needs. Treadmill-only or purely easy days produce weaker estimates.
Fix your max heart rate. An inaccurate max-HR setting biases the whole calculation. Set it from data if you can, not the “220 minus age” default.
Wear the sensor properly, or pair a chest strap. Optical wrist HR is the weakest link; a chest strap removes a big source of noise.
Compare like with like. Don’t compare your Garmin number to a friend’s Apple Watch number: different algorithms, not interchangeable.
Watch the direction, not the decimal. A one-point wobble is noise. A steady multi-month climb is signal.

Tomatoes, farmers market, healthy — illustrating How Accurate Is Your Watch's VO2 Max?

The takeaway

Your watch’s VO2 max is a reasonable, running-based estimate that leans low and gets less reliable the fitter you are: good for tracking your own trajectory, weak as a precise or cross-person score. If you want to understand why this number is worth watching at all, see how VO2 max predicts lifespan and all-cause mortality, and if you want to move the number rather than just measure it, our guide to raising your VO2 max with the right training covers the protocols that work. To sanity-check your wrist estimate against a field-test-based figure, run your numbers through the VO2 max calculator.