VO2 max on your watch: a useful fitness trend with a wide margin of error

VO2 max is the maximum rate at which your body can take up and use oxygen, expressed in millilitres per kilogram of body weight per minute (mL/kg/min). Measured properly, it means running or cycling to exhaustion with a mask that analyses every breath. Watches estimate it instead from heart rate, pace or power, and your profile. The estimate is useful. It is also less precise than the single decimal place on the screen suggests.

Why fitness matters

A 2009 meta-analysis in JAMA pooled 33 studies with 102,980 participants. Each 1 MET higher maximal aerobic capacity (one MET is 3.5 mL/kg/min, roughly a 1 km/h faster running speed) was associated with a 13 percent lower risk of death from any cause (relative risk 0.87) and 15 percent fewer cardiovascular events. People with low fitness, under 7.9 METs (about 27.7 mL/kg/min), had a 70 percent higher all-cause mortality than those with high fitness. These are observational associations, but they are large and consistent, which is why fitness is worth tracking.

How watches estimate it

Garmin's manuals say VO2 max estimates are provided by Firstbeat Analytics, with ratings from The Cooper Institute. A running estimate needs heart rate (wrist or chest strap) and an outdoor GPS run of at least 10 minutes reaching at least 70 percent of maximum heart rate; cycling estimates need a power meter and a steady, hard ride of at least 20 minutes. Garmin advises completing your user profile and setting your maximum heart rate, and warns that the first estimates may seem off while the watch learns your performance. Apple Watch produces a similar estimate, which it calls cardio fitness.

What the validation studies found

The pattern is clear. Estimates based on exercise data are accurate on average across a group, which is why researchers find them useful for population studies. For one person, the error band is wide: the limits of agreement in the meta-analysis span roughly 10 mL/kg/min either side even for the better algorithms. The Apple Watch studies found a systematic underestimate of about 6 mL/kg/min in fit, healthy adults. A note on dates: the publisher of the 2022 meta-analysis issued a correction in October 2026; the figures above are from the original abstract.

Where the fitness bands sit

Given the error bands above, a watch estimate near a boundary could fall in either band. In the 2026 Apple Watch study, 14 percent of people were placed in the right fitness percentile band and a further 34 percent were one band out.

What a good validation looks like

The INTERLIVE network of researchers that ran the 2022 meta-analysis also set out what future validation studies should report, covering six areas: the target population, the reference standard (a laboratory VO2 max test), the device measure itself, the testing conditions, data processing, and the statistical analysis. When a company quotes accuracy for a new watch, those are the questions to ask: tested on whom, against what, under which conditions.

Reading your number

• Use the trend. The same watch, the same kind of workout, over months. A steady rise means fitness is improving, whatever the absolute figure.
• Do not over-read the percentile. The 2026 study placed only 14 percent of people in the correct fitness percentile band.
• Help the algorithm. Garmin advises completing your profile and setting your maximum heart rate. Keep age and weight up to date.
• Expect weight to move it. VO2 max is expressed per kilogram, so losing weight raises it even if absolute aerobic capacity is unchanged. During rapid weight loss that is worth remembering.
• Want a precise number? A laboratory test with gas analysis is still the reference.

Improving it

Aerobic training raises VO2 max at any age, and the people who start least fit gain the most. The cardio and Zone 2 guide covers how much and how hard, and daily steps covers the walking research. If you are losing weight on a GLP-1, combining that with strength and aerobic training protects muscle and fitness.

Logging it

Peptide AI brings VO2 max estimates in from Garmin and Apple Health through wearable integrations and keeps them next to the training log and weight. Over months, the trend is the useful part.