Measuring VO2max: how reliable is your watch next to the lab?

Your watch does not measure oxygen. It estimates your VO2max from your heart rate, your pace and an algorithm, usually Firstbeat. A real measurement happens with a mask over your face, while a device analyses the air you breathe out.

That distinction sounds pedantic. It decides whether your figure is worth anything.

I once saw 4 points of difference between two watches in the same week. Neither was lying; they were simply calculating different things.

How do you measure VO2max? Four methods compared

Four practical routes exist: your sports watch, a field test such as the Cooper test, a submaximal cycle test at a sports physician, and a maximal lab test with breath-gas analysis. Only the last measures your oxygen uptake directly. The other three derive it from performance or heart rate.

Price and precision differ considerably.

The error margins are indications from validation studies, not a guarantee for your own reading.

One thing stands out: the gap between a watch and a field test is smaller than people assume. Both calculate, just from different inputs.

How accurate is the VO2max on your smartwatch?

Reasonable, as long as you are not a well-trained endurance athlete. A systematic review from December 2025 filtered 252 publications down to thirteen usable studies. In seven of them, wearables proved valid or acceptable for estimating VO2max against a treadmill lab test (PMID 41477023).

In elite endurance athletes, usefulness remained questionable.

Two details from that review are practically useful. Most studies looked at Garmin watches with a chest strap and the Firstbeat algorithm, so not wrist measurement alone. And the valid estimates came from submaximal runs, not from all-out efforts.

That last point explains why your figure sometimes drops after a hard week. Your algorithm only saw heavy, fatigued sessions and drew the wrong conclusion from them.

The same principle applies to your heart-rate data, where an even bigger error hides in the default settings: see why your heart rate zones are probably wrong.

How do you run the Cooper test yourself?

You run as far as you can in twelve minutes on a flat track and note your distance in metres. That distance goes into the formula: VO2max is roughly the distance minus 505, divided by 45. Warm up calmly first, and pace yourself, because almost everyone starts too fast.

An example makes it concrete.

Say you run 2,600 metres. That lands you around 46.5 ml/kg/min. Run 2,200 metres and you get about 37.7. So 400 metres is worth almost nine points, which shows how sensitive the formula is to a bad day.

Always run it under the same conditions. Same track, comparable weather, comparable rest beforehand.

If you have symptoms during effort, such as chest pain, dizziness or breathlessness out of proportion to the work, do not run a maximal test. The Dutch Hartstichting is clear on this: those symptoms belong with your GP or a sports physician. What a clinical cycle test does and does not see sits in the exercise stress test explained.

Is a lab test worth it?

Not every year for most athletes, but once is worth it. A lab test gives you your heart-rate and lactate thresholds alongside your VO2max, and those thresholds drive your training zones far more directly than a percentage of an estimated maximum. That is where the money goes.

So you are mostly buying zones, not one number.

Joyner and Coyle showed back in 2008 that endurance performance rests on three pillars: VO2max, lactate threshold and efficiency (PMID 17901124). A lab test touches all three in one morning. Your watch touches at most one, and even then indirectly.

Which of those pillars drives your race time hardest sits in lactate threshold or VO2max.

What does your VO2max actually mean?

It is the maximum amount of oxygen your body can take in and use per minute, expressed per kilo of body weight. The higher the number, the more aerobic work you can produce. What counts as good for your age and sex sits in the tables in what is a good VO2max.

The number is also a health measure, not purely a sports figure.

The American Heart Association called fitness a clinical vital sign in 2016, because low fitness is associated with a higher risk of cardiovascular disease and mortality (PMID 27881567). That is a group-level association. It says nothing about whether your 43 today is good or bad news.

Which number should you follow?

Follow the direction, not the reading. Pick one method, repeat it under equal conditions, and look at the line after three months. Switching between watch, Cooper test and lab test mostly produces confusion, because you are comparing three different calculations.

My own choice: the watch as a weekly signal, the Cooper test each quarter as a check.

If that line stays flat while your training continues, your oxygen transport is the next place to look. Iron, haemoglobin, thyroid and inflammation markers sit together in InsideTracker. Why that side can set your ceiling sits in cardio fitness and your blood.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Zeleznik Mezan L. Accuracy of wearables for determining the maximal oxygen uptake and lactate threshold: a qualitative systematic review. Frontiers in Sports and Active Living, 2025;7:1707991. PMID 41477023.
• Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. Circulation, 2016;134(24):e653-e699. PMID 27881567.
• Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. The Journal of Physiology, 2008;586(1):35-44. PMID 17901124.
• Hartstichting. Exercise and your heart rate.