VO2 Max Test: The Cheap Estimates Sorted People Into Three Fitness Bands and Got It Right Half the Time
Somebody checked whether the cheap ways of working out your VO2 max actually work. They compared fifteen of them against the real thing, across 4,871 tests. [1]
The task was not even hard. Just sort each person into a fitness third — bottom, middle or top. Three buckets.
The submaximal equations got it right 51% of the time. The maximal ones managed 59%. [1]
Guessing gets you about 33%, because there are three buckets. So the cheap estimate is better than a coin toss and nowhere near a measurement, and the researchers say so in their own conclusion: “a low degree of accuracy in categorizing participants into fitness tertiles”. [1]
What a real VO2 max test actually is
A mask over your face, a treadmill or a bike, and a machine measuring the oxygen going in and the carbon dioxide coming out, breath by breath, while the effort is increased until you cannot continue.
VO2 max is the most oxygen your body can take in and use in a minute, per kilogram you weigh, flat out. The reason the test hurts is that the number is defined at your limit, so the only way to find it is to go there.
How does the lab know you actually got there? The standard check is the respiratory exchange ratio — the amount of carbon dioxide you breathe out divided by the oxygen you take in. At rest it sits near 0.8; once you are genuinely at your ceiling it goes above 1.0, and the large American reference database requires 1.0 or higher before it will accept a test as maximal. [3]
That criterion is the thing no estimate has. A formula cannot tell whether you truly emptied the tank. It assumes you did.
The fifteen shortcuts, ranked
| Method | Sorted correctly | Range |
| Guessing (three bands) | ~33% | the floor |
| Submaximal exercise equations | 51% | 37–58% |
| Maximal exercise equations | 59% | 43–76% |
| Directly measured, mask and gas analyser | — | this is the standard |
Notice the ranges. The best maximal equation got 76% and the worst got 43% — and unless you know which one your gym used, you do not know which of those you are holding. [1] Across all fifteen, the agreement statistic ran from 0.07 to 0.89, where 1.00 would be perfect. [1]
And the one on your wrist
Your watch does not run any of those fifteen equations. It runs its own, from your heart rate and your pace.
When the Apple Watch Series 7 was put against laboratory equipment, agreement came out at 0.47, which the authors classify as poor, with a typical error of 8.85 mL/kg/min. [2] Nineteen people, which is small, and the sibling page goes through that study properly, including the fair defence of it.
For scale: 8.85 is roughly the distance from the middle of the pack to the top tenth for a man in his forties.
So do you need one?
Mostly no, and this is the part the answer turns on: what would you do differently?
The number is worth having when it changes a decision — a clinical assessment, a surgical risk workup, a serious training block built around measured thresholds. In those settings the researchers argue directly for the mask, because sorting people into risk categories on a formula that is right half the time is not good enough. [1]
Outside those settings, the honest use of an estimate is as a trend, not a score. A device that is consistently wrong in the same direction can still show you a change, because the error largely cancels. What it cannot do is tell you your percentile, and it cannot be compared against somebody else’s device.
And the thing that raises the number does not require knowing it. Training moves VO2 max by about as much as a decade of ageing takes off it, whether or not anyone measured you. Walking more is the version of this with the lowest barrier.
What the number is actually good for
The reason anyone cares is one of the largest associations in this field: an overview of 26 systematic reviews, 199 cohorts and 20.9 million observations, puts the risk of dying for high-fitness people at roughly half that of low-fitness people. [4]
That evidence is observational and its own reviewers grade the certainty from very low to moderate. [4] Here is how we weigh that kind of evidence. It compares fitter people with less fit people; it does not promise that moving your own number buys you the difference.
Which is the useful way to end this. Fitness is one of the few things measured about you that you can also change — unlike grip strength, which predicts a lot and does not respond to squeezing a gripper. The measurement is optional. The training is the part that does anything.
