Why your Apple Watch, Garmin and Samsung all show a different VO2 max

You finish the same loop you always run. Your Garmin flashes a VO2 max of 52 and a fitness age five years below your real one. Your partner, running beside you on an Apple Watch, checks Cardio Fitness and sees 46. You did the same run, at the same pace, in the same air. The watches disagree by six points.

Neither one measured anything. They both guessed, from different rules, and the guess is more useful than it looks once you know what it is doing.

Nobody’s watch measures VO2 max

VO2 max is the most oxygen your body can take in and use per minute, per kilogram of body weight, at full effort. The real test involves a mask, a treadmill that keeps getting harder, and a lab technician watching you run to exhaustion. That number is the gold standard, and your watch has never taken it.

What a watch has is your heart rate and your speed. The core idea is simple: at a given submaximal pace, a fitter person’s heart works less hard. Feed enough pace-and-heart-rate pairs into a model and it can estimate the VO2 max that would explain them. Every wearable brand does a version of this, and every one keeps its exact model private.

Why the estimates land in different places

The estimate is only as good as its inputs and its algorithm, and both differ by brand.

Read the middle rows. Apple wants you outdoors with GPS, so an all-treadmill month leaves its estimate stale. Garmin’s Firstbeat model sharpens when you give it hard efforts and can sulk if every run is a plod. Each brand also folds in your age, weight and sex, and a wrong weight in a profile quietly shifts the result. Same body, three intake forms, three answers.

Fitness age is VO2 max wearing a costume

The number that makes people happiest or most annoyed is fitness age, and it is not a new measurement. It takes your estimated VO2 max and maps it onto the population curve for your sex, then reports the age at which your number would be average. Strong cardio fitness for a 40-year-old comes back as a fitness age in the low 30s.

The idea comes out of research at NTNU in Norway, where a large study of cardiorespiratory fitness produced a model that estimates VO2 max from things like resting heart rate, waist and activity, then expresses it as an age. Garmin’s version leans on the same lineage. It is a genuinely motivating way to read a hard-to-picture number, as long as you remember it inherits every bit of wobble from the VO2 max underneath it.

Why the number is still worth watching

Here is the part that makes the guess worth keeping. Of all the numbers on your wrist, VO2 max may be the one most worth improving. A large 2018 study in JAMA Network Open followed over 120,000 people through treadmill testing and found cardiorespiratory fitness tracked with living longer across the whole range, with the least fit facing the steepest risk. Higher measured fitness, more years.

Your watch is not running that study on you. But if its estimate climbs three points over a training block, the real thing underneath it almost certainly moved in the same direction, and that direction is the one associated with the good outcomes. That is why the trend matters and the daily digit does not.

This is the same lesson as the recovery scores: a private, individualised index is superb for tracking yourself and close to meaningless held up against someone else’s device. It is also why a fair cross-brand competition is built on plain units like active minutes instead.

What to do with this

1. Pick one device for VO2 max and stay on it. The number only means something relative to its own history, so switching wrists resets the story.
2. Feed it what it wants. Apple needs outdoor GPS efforts; Garmin wants the occasional hard run. An estimate starved of good data drifts.
3. Read the season, not the day. One hot, dehydrated or altitude run drops the estimate with no real change in you. Judge it over weeks.
4. Fix your profile. A stale weight or wrong age silently bends both VO2 max and the fitness age built on it.

That split, between a personal number you improve and a shared number you compete on, is exactly how Fit, the cross-brand fitness competition app, is built. Your fitness age is yours to chase against your own past, and your crew is ranked on the units that survive the jump between an Apple Watch, a Garmin and a Samsung. Where Fit stops: it will not lab test your aerobic ceiling, and if you want the deepest VO2 max analysis, Garmin and its Firstbeat lineage still go further.