What is VO2 max? What your watch's number means and how far to trust it

You open the Health app on a Tuesday evening. Your cardio fitness reads 38. A month ago it read 41.

Three points in four weeks. Is your fitness draining away? Should you be worried?

Probably not. But to know for sure, you need to understand what the number is. Because it is not a measurement. It is a calculation worked out backwards, and it carries a margin of error that is larger than most people realise.

What VO2 max actually is

VO2 max is the largest amount of oxygen your body can take up and use per minute when you are working as hard as you possibly can. It is usually given in millilitres of oxygen per kilogram of body weight per minute, written as ml/kg/min. The higher the number, the more work your engine can do before it hits the ceiling.

What makes the measure so useful is that it captures the whole chain at once. Your lungs take the oxygen in. Your heart and blood carry it. Your muscles use it. That is why VO2 max says more about your fitness than any single component. Resting heart rate, for example, is related to fitness but does not measure it, as the guide to resting heart rate explains.

The proper way to find your VO2 max is an exercise test with a mask, on a treadmill or a bike. The speed or resistance goes up in stages until you cannot carry on. Meanwhile a gas analyser measures the oxygen in the air you breathe in and out. It is expensive, it takes time and it requires you to go completely flat out. Most people in midlife have never done one.

Why the number is worth caring about

Fitness is among the risk markers with the strongest and most consistent support in health research. Three sources are enough to see why.

In 2009 a meta-analysis in the journal JAMA pooled 33 cohort studies covering just over 100,000 healthy men and women. Fitness was measured in METs, where one MET corresponds to roughly 3.5 ml/kg/min. For every MET of higher fitness, the risk of dying during follow-up was 13 percent lower. The risk of cardiovascular events was 15 percent lower. The study itself translated one MET into running 1 km/h faster.

In 2018 JAMA Network Open published a study of 122,007 patients who had done a treadmill exercise test at an academic medical centre. They were followed for a median of 8.4 years. Those with the lowest fitness for their age and sex had roughly five times the adjusted mortality of the fittest of all. Being below average rather than above it carried an increase in risk of the same size as smoking or diabetes.

In 2016 the American Heart Association issued a scientific statement arguing that fitness should be assessed routinely in healthcare, as a clinical vital sign. The statement notes that low fitness is potentially a stronger predictor of mortality than established risk factors such as smoking, high blood pressure, high cholesterol and type 2 diabetes.

Your watch does not measure. It works backwards.

Now for what makes the watch's number something other than the lab's.

Apple describes its method fairly openly. The watch estimates VO2 max by measuring how hard your heart is working during an outdoor walk, run or hike started in the Workout app. According to a 2025 validation study, heart rate is combined with speed, distance and elevation from satellite positioning, along with details you have entered yourself: age, sex, height and weight. Apple says the watch also takes into account medications that affect heart rate. Indoor workouts do not count.

The principle behind this kind of estimate is an old one. At any given pace, a fit heart beats more slowly than an unfit one. If you know how your heart rate rises with your pace, you can work out roughly how much oxygen you could have taken up had you kept going all the way to the ceiling. This is called a submaximal estimate. You never have to go flat out. Apple does say, however, that the calculation needs some notion of where your maximum heart rate lies.

The conditions in Apple's technical paper and support page show how sensitive the calculation is to circumstances.

Two consequences are worth pausing on.

The first concerns medication. Apple itself says that conditions or medications that limit your heart rate can cause the estimate to come out too high. If you take beta blockers, for example, that needs to be recorded in your health details, or the watch is calculating on the wrong assumptions.

The second concerns anything that raises your heart rate at the same pace. If the number is built on how your heart rate responds to your pace, such things end up in the number, because your heart rate climbs while your pace stays put. Apple's own paper lists dehydration, caffeine, extreme heat and a recent arrival at high altitude as factors that can lead to underestimates. Why heart rate creeps upwards during a steady session is covered in the guide to zone 2.

How large the error is against a real maximal test

The manufacturer's own figure first. In its 2021 technical paper, Apple reports that in its validation group of 221 people the estimate was on average 1.4 ml/kg/min away from the reference value. That sounds impressive.

Then look at what the reference was. Apple did not compare against a maximal test in which participants went all out. The reference value was calculated from heart rate and oxygen uptake in the submaximal part of exercise tests, projected out to an age-predicted maximum heart rate. In other words, the watch was compared with another estimate, albeit a more controlled one. The spread around the average was also more than three times the size of the average error itself: the standard deviation was 4.7.

The question you actually have is a different one. How close does the watch get to a real maximal test? Two independent research groups have tested exactly that with the Apple Watch.

Mean error here is the mean absolute percentage error. Both studies found that the watch read too low on average. The average is still not the most important part, though. The spread is.

In the 2025 study, 95 percent of the differences between watch and lab would be expected to fall within a range running from just over 6 points above the lab value to just over 18 points below it. That is a window of more than 24 points. The 2024 study found poor reliability, and found that the watch, like other watches, overestimated people with poor fitness and underestimated people with excellent fitness.

The whole field at once

A meta-analysis in Sports Medicine in 2022 examined 14 validation studies of VO2 max from a range of consumer wearables. The researchers split the devices into two groups according to how they calculate.

The conclusion cut both ways. Estimates based on exercise perform best and look suitable for measuring fitness at population level. For the individual the error is large, the researchers write, and for sport or clinical use the methods still need to improve.

Other brands show a mixed picture. A German study from 2019 tested five wrist-worn devices on 24 people. Only a Garmin Forerunner 920XT came in under 10 percent mean error for VO2 max, at 7.3 percent, and it underestimated. In an American study from 2025, a Garmin fēnix 6 had a mean error of 7.1 percent against the lab in 19 people, which met the researchers' own criteria. A 2025 review found that most studies involved Garmin watches paired with a chest strap. That is worth knowing, because heart rate from the wrist has its own sources of error. Those are covered in the guide to chest strap versus wrist heart rate.

So what do the meta-analysis's ten points either way mean in real terms? In the international FRIEND reference registry, the median man in his twenties scored 49.5 and the median man in his seventies scored 30.8. A margin of error of ten points is therefore more than half that gap. Ten points is also nearly three METs. In other words, a single reading can put you several METs out on the very scale the health research is built on.

The number is good for one thing: your own trend

All of the above sounds like a reason to ignore the number. It is not. It is a reason to use it for the right job.

The error against the lab is one thing. How much the watch fluctuates against itself is another. According to Apple's own paper, the typical spread between one person's estimates was 1.2 ml/kg/min. For the tenth of people who fluctuated most, it was 2.6 or more. A watch that is off, but off in roughly the same way each time, can still show which direction you are heading.

Two caveats belong here. That stability is the manufacturer's own measurement. And no independent study has yet tested how well the watch picks up a real change over months, and a 2025 review explicitly calls for longitudinal studies that follow wearables' accuracy over months or years. Reading the trend is the most sensible way to use the number, not a proven one.

With that said, back to Tuesday evening. A number that has gone from 41 to 38 in a month is not far off the watch's own noise for many users. It is not a verdict. It is a question to revisit in three months.

How to read the number

1. Compare only with yourself, on the same watch. Never with a friend, never across brands, and never across a change of watch model without starting over.
2. Look at months, not weeks. Week-to-week swings of a couple of points tell you too little.
3. Keep your details up to date. The number is calculated per kilogram of body weight. If your weight changes, the number changes, even if the engine has not. Medications that affect heart rate should be entered.
4. Give the watch clean workouts. Flat ground, outdoors, an even pace. A hot summer's day tells you less than a cool autumn morning.
5. Keep your own test alongside. Every so often, walk or run the same loop at the same easy heart rate. If you get round faster at the same heart rate, your engine has improved, whatever the watch says.

After 40: the curve slopes, but the level can be moved

VO2 max falls with age. The questions are how fast, and whether anything can be done about it.

The clearest picture comes from studies that follow the same people over time. In the Baltimore Longitudinal Study of Aging, healthy adults were tested repeatedly on a treadmill over a median follow-up of 7.9 years. The results were published in Circulation in 2005. The decline was 3 to 6 percent per ten years in people's twenties and thirties. After that the rate picked up with each decade, reaching more than 20 percent per ten years from the seventies onwards. From the forties upwards, the decline was larger in men than in women.

One finding is worth stating plainly: the rate was roughly the same however physically active participants said they were. That sounds discouraging, but it says something else too. The slope of the curve is hard to change. The height it starts from is not.

That height is where training studies come in. A meta-analysis in Sports Medicine in 2015 pooled 28 controlled studies with 723 healthy adults aged 18 to 45. Compared with control groups who did not train, steady endurance training raised VO2 max by an average of 4.9 ml/kg/min. Interval training raised it by 5.5. Intervals gave a little more, then, but both approaches produced large improvements. Participants averaged 25 years of age, so the figures cannot be transferred directly to a 45-year-old. How to set intervals up in practice is covered in the guide to running intervals.

The Swedish picture is worth knowing about. A study in the Scandinavian Journal of Medicine & Science in Sports in 2019 tracked the fitness of 354,277 Swedish working adults who took part in occupational health screenings between 1995 and 2017. Fitness per kilogram of body weight fell by 10.8 percent over the period, with around a third explained by rising body weight. The share with low fitness, below 32 ml/kg/min, rose from 27 to 46 percent. Two of the authors were employed by HPI Health Profile Institute, which manages the database and the test method, and a third received fees from the company. It is also worth noting that the whole Swedish dataset rests on an estimate from a submaximal bike test, not on maximal tests. So estimates are not to be dismissed. They are simply estimates.

The practical conclusion for anyone in midlife: you cannot stop the curve, but you have a big say in where on it you sit. The fitness you build now is the fitness you have to decline from later. It also fades more slowly than its reputation suggests, as the guide to how fast you lose fitness explains.

When the number is not a training question

The Apple Watch can send a notification that your cardio fitness is low. According to Apple's technical paper, people aged 20 to 59 receive it when their estimate falls in the lowest fifth for their sex and age in the FRIEND registry. From 60 onwards, fixed thresholds are used: 18 ml/kg/min for men and 15 for women. You have to switch the notification on yourself.

A notification like that is not a diagnosis. Given the margins of error described above, it can be wrong in either direction. It is still a reasonable prompt to look at how much you move. The number alone, however, does not decide whether you should talk to a doctor. How you feel when you exert yourself tells you more.

What this means when you open the Health app tonight

The summary fits in a few lines.

VO2 max is one of the strongest markers of long-term health there is. Your watch does not measure it but works backwards from heart rate and pace. Against real maximal tests, the error has been large enough to put an individual several METs out. The same watch does appear fairly stable against itself, which makes the trend over months the most useful thing you can get from the number.

Follow the curve, not today's figure. Give the watch flat outdoor workouts. Build the engine with training you can actually keep repeating. A number that drops for a month is data, not a failure.

Frequently asked questions

What is VO2 max?

VO2 max is the largest amount of oxygen your body can take up and use per minute at maximal effort, usually given in millilitres per kilogram of body weight per minute. It is regarded as the best single measure of fitness, because it captures the whole chain from lungs and heart to muscles. A 2009 meta-analysis of 33 cohort studies in JAMA found that each MET of higher fitness, roughly 3.5 ml/kg/min, went with a 13 percent lower risk of dying during follow-up.

How does a watch calculate VO2 max?

It does not measure your oxygen uptake. It estimates it from how your heart rate responds to your pace during an outdoor walk, run or hike, together with your age, sex, height, weight and any medication that affects heart rate. The Apple Watch requires good GPS, a gradient under 5 percent and a rise in heart rate of about 30 percent of the range between resting and maximum heart rate. Indoor workouts do not count. According to Apple, conditions or medications that limit heart rate can make the estimate too high.

How accurate is Apple Watch VO2 max?

In two independent studies against real maximal tests, the watch read too low on average, with a mean absolute percentage error of 15.8 percent for the Series 7 and 13.3 percent mainly for the Series 9 and Ultra 2. In the second study, 95 percent of differences would be expected to fall between just over 6 points above and just over 18 points below the lab value. Both studies were small, and participants were mostly young and fit. Apple's own paper reports an average error of 1.4 points, but against a submaximal reference rather than a maximal test.

What is a good VO2 max for my age?

It depends on age and sex. In the international FRIEND reference registry, the median for ages 20 to 29 was 49.5 ml/kg/min for men and 40.6 for women, while the median for ages 70 to 79 was 30.8 for men and 25.0 for women. The registry showed an average decline of 9 percent per decade. Bear in mind that a watch's number can be several points off for an individual, so comparing it with a table of norms is rough at best. Your own trend on the same watch tells you more.

How can I improve my VO2 max?

With regular endurance training. A 2015 meta-analysis of 28 controlled studies in Sports Medicine found that steady endurance training raised VO2 max by an average of 4.9 ml/kg/min and interval training by 5.5, compared with people who did not train. Intervals gave a little more, but both approaches produced large improvements. Participants were aged 18 to 45. Choose the approach you will actually keep repeating week after week.