Because they are two different estimates, and Apple’s tends to read low. Neither watch measures oxygen. Both infer VO2 max from your heart rate and pace, using different methods and different workouts, so they rarely agree. A gap of several points is normal.
How does each watch estimate it?
Apple Watch looks at how your heart rate responds to steady effort during Outdoor Walk, Outdoor Run and Hiking workouts on fairly flat ground. You do not need to push hard, and ordinary walks count.
Garmin uses a method from Firstbeat that relates your pace to your heart rate during runs, and power to heart rate on the bike.
Different inputs, different maths, different result. If most of your Apple estimates come from walks and easy runs, that alone can explain part of the gap.
Which one is right?
Neither is a lab test. A true VO2 max is measured with a mask, to exhaustion.
For Apple Watch there is published evidence. Two independent studies compared it with lab measurements and both found it read low, by about 6 ml/kg/min on average, with wide differences between individuals. One of them also found that it tends to overestimate people with low fitness and underestimate people with high fitness, which is the group most runners asking this question belong to.
Garmin’s estimate has its own error, and it also varies from person to person. The honest summary is that both are rough, and Apple’s is more likely to be the lower of the two for a trained runner.
Does the difference matter?
Not for training. What you can use is the direction over months, and both watches show that. A number that climbs from 44 to 47 on the same watch tells you more than the gap between two watches.
Two things do matter:
- Do not mix sources. If you switched watches, treat it as a new baseline. Your fitness did not drop six points on the day you changed devices.
- Give the watch good data. Apple’s estimate needs outdoor workouts on flat ground with a good heart-rate signal. See Why is my VO2 max not updating on Apple Watch?.
Medication that limits your heart rate, such as beta blockers, pushes the estimate the other way, and can be recorded under Health Details in the Health app.
What does Kiiro show?
Kiiro shows the VO2 max values your Apple Watch saves in Apple Health, as a trend over time next to your resting heart rate and HRV. It does not calculate its own value, so the number matches the Health app. See VO2 max trends in Kiiro.
Related questions
- Why is my VO2 max not updating on Apple Watch?: Apple Watch only estimates VO2 max during outdoor walks, runs and hikes on fairly flat ground, with good GPS and heart-rate signal and enough effort. Here is what to check.
- Does Apple Watch have a training status like Garmin?: Not under that name. Apple Watch has Training Load, which tells you whether your last week was more or less than usual, but no label such as Productive or Detraining. Here is what it offers and how to get a status.
