VO₂ max estimates: Garmin vs Apple vs Coros — why they differ
Switch watches and your VO₂ max can jump several points overnight — not because your fitness changed, but because a different model started guessing.
All of them are estimates, from different models
A real VO₂ max needs a maximal-effort lab test with gas analysis. No watch does that. Each brand estimates it from the relationship between your heart rate and your pace during ordinary runs, through a proprietary model — and the models differ in their assumptions, their data windows and their calibration. So Garmin, Apple and Coros can each look at the same runner and return numbers several points apart, all internally consistent and none a measurement.
Why switching devices resets it
Because the number lives inside a model, moving to a new watch starts a new model from scratch — often with a settling period as it learns your heart-rate-to-pace relationship. The step change you see on day one is the handoff between two estimators, not a fitness event. Reading it as a sudden gain or loss is the commonest VO₂ max misinterpretation.
Read the slope, on one device
What VO₂ max is good for is direction over months on a single device: a slow rise through a training block, a dip through illness. Those slopes are honest. The absolute number, and any comparison to a friend’s watch or a normative chart from a different method, carries a model’s bias you cannot see or correct.