A smartwatch does not measure VO2 max — it estimates it. Laboratory VO2 max testing requires a breathing mask and a treadmill or bike protocol run to exhaustion; your watch instead predicts the value from how your heart rate responds to a known workload, such as your running pace or cycling power. Published validation research and manufacturer white papers generally put wrist-based estimates within roughly 5-10% of lab results for most people, with larger errors for very fit athletes, beginners, and anyone whose heart rate data is noisy. That makes the number good for tracking trends over months, and less trustworthy as a single absolute figure.
- Watches predict VO2 max from heart rate vs. pace or power, not from measuring oxygen
- Typical error is around 5-10% versus lab testing, and can be larger at the extremes of fitness
- The trend over weeks and months is far more meaningful than any single reading
- Accurate profile data (age, weight, max heart rate) and outdoor GPS workouts improve the estimate
What VO2 max actually measures
VO2 max is the maximum volume of oxygen your body can use per minute during intense exercise, usually expressed in milliliters per kilogram of body weight per minute (mL/kg/min). It reflects how well your heart, lungs, blood, and muscles work together, and it is one of the most studied markers of cardiorespiratory fitness. Research summarized by organizations like the American Heart Association links higher cardiorespiratory fitness with lower long-term risk of cardiovascular disease and early death — which is why Apple, Garmin, Samsung, and others surface the metric so prominently.
The gold standard is a graded exercise test in a physiology lab, where a metabolic cart analyzes every breath as the workload increases until you can’t continue. Nothing on your wrist replicates that.
How a smartwatch estimates VO2 max without a mask
Wearables use a well-established shortcut: the relationship between heart rate and external workload is roughly linear, and the slope of that relationship predicts aerobic capacity. In practice, the watch combines:
- Optical heart rate from the wrist sensor (or a paired chest strap)
- Workload data — GPS pace for running and walking, or power for cycling
- Profile data — age, sex, weight, and your set or detected maximum heart rate
Garmin’s estimates come from Firstbeat Analytics algorithms; Apple calls its version Cardio Fitness and requires outdoor walks, runs, or hikes recorded with the Workout app; Samsung and Fitbit use comparable models. If you run a given pace at a lower heart rate than before, the algorithm concludes your aerobic capacity has improved.
So how accurate is it, really?
Firstbeat’s own white papers claim roughly 95% accuracy (about a 5% error) against lab testing under good conditions. Independent validation studies published in sports science journals paint a slightly less flattering but still respectable picture: mean errors commonly land in the 5-10% range, with individual readings occasionally off by more. A few consistent patterns show up across the research and in expert reviews:
- Watches tend to compress the range. Estimates often run high for less-fit users and low for highly trained athletes, pulling everyone toward the middle.
- Running-based estimates beat resting or walking estimates. Higher-intensity outdoor GPS runs give the algorithm the cleanest workload signal.
- Consistency is good even when accuracy isn’t. A watch that reads 3 points high tends to keep reading 3 points high, so changes over time are still meaningful.
- A wrong max heart rate setting skews everything. Many watches default to the crude 220-minus-age formula, which misses badly for plenty of people.
This mirrors what we’ve found across other wrist metrics — see our looks at smartwatch calorie counts and SpO2 accuracy: derived estimates are directionally useful, not clinical measurements.
How the major platforms compare
| Platform | Metric name | Primary data source | What it needs from you |
|---|---|---|---|
| Apple Watch | Cardio Fitness | Heart rate + GPS during outdoor walk/run/hike | Outdoor workouts via the Workout app; accurate Health profile |
| Garmin | VO2 Max (Firstbeat) | Heart rate vs. pace or cycling power | 15+ min outdoor runs; correct max HR setting |
| Samsung Galaxy Watch | VO2 max / Cardio load tools | Heart rate + GPS during runs | Outdoor running workouts in Samsung Health |
| Fitbit / Google Pixel Watch | Cardio Fitness Score | Resting heart rate model, refined by GPS runs | Runs with GPS for the tighter estimate |
How to get a more reliable number
What the number actually means for you
Most platforms grade your estimate against population norms for your age and sex — labels like “below average,” “good,” or “superior.” Broadly, higher is better: a typical mid-30s adult might see values in the 35-45 mL/kg/min range, while trained endurance athletes can exceed 60. The practical takeaway isn’t the label, though — it’s the direction. A number that climbs as you train consistently indicates your aerobic fitness is genuinely improving, and that’s the signal the estimate captures well.
Some watches, including Apple Watch, may flag a persistently low Cardio Fitness level. That can be a useful prompt to talk to your doctor, but it is not a diagnosis of anything — the same caution that applies to smartwatch AFib notifications.
Frequently asked questions
Why is my VO2 max different on two different watches?
Each brand uses its own algorithm, workout requirements, and max heart rate assumptions, so a Garmin and an Apple Watch can disagree by several points on the same wrist. Pick one platform and follow its trend rather than comparing absolute numbers across devices.
Why did my VO2 max drop even though I’m training?
Short-term drops usually reflect elevated heart rate from heat, fatigue, stress, illness, or dehydration — not lost fitness. Slower-than-usual easy runs can also nudge the estimate down on some platforms. Look at the multi-week trend before drawing conclusions.
Is a chest strap more accurate than the wrist sensor for VO2 max?
It can help. The estimate is only as good as its heart rate input, and chest straps are generally more reliable during hard intervals, in cold weather, and for people with tattoos or smaller wrists. Cleaner input data typically means a steadier, more plausible estimate.
Can I improve my VO2 max, and how fast?
Yes — consistent aerobic training, especially a mix of easy volume and some higher-intensity intervals, can raise it. Newer exercisers often see measurable gains within 6-12 weeks; improvements come more slowly the fitter you get. Genetics and age also set part of the ceiling.