How accurate is Garmin VO2 max (and Apple Watch)?

Your watch shows a VO₂ max with a decimal point, which makes it look like a measurement. It is an estimate, made from heart rate and pace during ordinary workouts. Here is how Garmin and Apple make it, what the validation studies found when they compared it with a lab test, and how to get a reading you can trust for trends.

How does Garmin estimate VO₂ max?

Garmin's estimate comes from Firstbeat Analytics. The watch compares your heart rate with your speed during outdoor runs, or with power on a bike, and fits your fitness to that relationship. According to Garmin's own manual, a running estimate needs an outdoor run with GPS of at least 10 minutes above 70% of your maximum heart rate; a cycling estimate needs a power meter and 20 minutes of steady hard riding. Garmin also says the first values may look off until the watch has learned from a few runs, and that a complete profile and a correct maximum heart rate improve accuracy.

The colour rating beside the number, from poor to superior, comes from The Cooper Institute norms, not from the lab registries in our percentile tables.

Free tool: Do one home test, type the result, read your estimate. Open the VO2 max calculator →

How does Apple Watch estimate VO₂ max?

Apple calls it Cardio Fitness. Apple's support page says it is estimated during Outdoor Walk, Outdoor Run and Hiking workouts from the heart and motion sensors, plus passive measurements, and it takes age, sex, weight, height and heart-rate-affecting medications into account. A first estimate needs at least 24 hours of wear and several qualifying workouts. The supported range is 14 to 65 ml/kg/min, and the fitness levels are shown to users aged 20 and over. Apple notes that conditions or medications that limit heart rate may cause an overestimate.

PeakVital does this from your own data. Three tests. One number. Your real age. Join the free beta →

What did the validation studies find?

The broader picture comes from a 2022 meta-analysis of 14 validation studies. Wearables that estimate VO₂ max during exercise were close to the lab on average, with a bias of almost zero, but individual readings could be about 10 ml/kg/min off in either direction. Devices that estimate from resting data overestimated by about 2 ml/kg/min on average, with even wider spread. The authors' conclusion: good enough for groups, not yet precise enough for individuals in sport or clinical use.

So how accurate is it for you?

Three practical conclusions follow from those studies:

• The trend is more reliable than the number. Systematic bias cancels out when you compare your watch with itself over months.
• Expect it to read a little low. In the Garmin and Apple studies above, the average error was an underestimate. A watch value a few points under a lab result is normal.
• Very fit people are estimated worst. In the Forerunner 245 study, error roughly tripled in the highly trained group. If your number is already high, a lab or field test is the better check.

How do you get a better reading?

1. Set your real maximum heart rate. The estimate leans on it. Take it from a recent all-out effort rather than 220 minus age.
2. Use a chest strap if you have one. Wrist sensors lose accuracy at high intensity and in the cold; Garmin supports compatible chest monitors.
3. Give it qualifying runs. Steady efforts on flat routes with good GPS. Hilly trail runs, intervals and runs in heat raise heart rate for reasons other than fitness.
4. Exclude what distorts it. Garmin lets you turn off VO₂ max recording for trail and ultra runs.
5. Read the monthly trend. A drop of two points after a hot week is noise; a drop over two months is a signal.

For a cross-check, run a Cooper or Rockport test at home every six weeks and see whether it moves the same way as the watch.

How PeakVital uses a watch estimate

PeakVital can take the VO₂ max your wearable reports or run its own guided field test, and it keeps the method fixed, so the six-weekly trend compares like with like. It is coming soon for Android and Wear OS, in private beta.

Hen Eytan · Founder, UPeak

Builds the UPeak apps and writes up the research behind them.