Of all the numbers a watch reports, VO₂ max (often written VO2 max) is the one most worth understanding, and the one most often misread. It predicts long-term health remarkably well. Your watch's version of it is also, for you personally, quite approximate.

Both of those things are true at once, and holding them together is the whole skill.

Key takeaways

• VO₂ max measures how much oxygen your body can use at full effort, which is a good proxy for overall cardiovascular fitness.

• In 122,007 people, the least fit had five times the risk of dying compared with the fittest, a larger gap than the study found for smoking or diabetes against their own comparison groups (Mandsager et al. 2018, JAMA Network Open).

• Watch estimates average out well across a group but run loose for any one person: individual estimates landed as much as 10 points above or below the true value (Molina-Garcia et al. 2022, Sports Medicine).

• Read yours as a trend line, never as a measurement.

What does VO₂ max actually measure?

It's the maximum amount of oxygen your body can take in and use during hard exercise, expressed per kilogram of body weight per minute. Getting oxygen from air to working muscle involves your lungs, heart, blood and muscles all cooperating, so a single number ends up summarising the whole chain.

That's why it travels under the name "cardiorespiratory fitness," and why it turns out to predict so much.

Why does VO₂ max matter so much?

Because the association with living longer is unusually strong.

Researchers followed 122,007 people who had done treadmill testing, recording more than 13,000 deaths over two decades. Compared with the fittest group, those in the lowest fitness group had about five times the risk of dying from any cause (Mandsager et al. 2018).

For scale, here is how being unfit compared with familiar risk factors in that same study:

Two things stand out. Each row compares against its own reference group, so these are not head-to-head rankings, but the size of the fitness gap is striking even so: the least-fit-versus-elite gap is far larger than the smoking gap. And the benefit kept going at the top. Even the fittest group still showed a further reduction against the merely high-fitness group, with no ceiling where more fitness stopped helping.

These are population associations, not predictions about any one person. But they're consistent enough that fitness is worth taking seriously as a health signal rather than an athletic one.

What is a good VO₂ max for my age?

It falls with age, and the reference values are well established. From a registry of 7,783 maximal treadmill tests, these are the median values in mL/kg/min (Kaminsky et al. 2015):

That works out to roughly a 10% decline per decade across the middle of life. But the honest picture is less tidy: a study that followed the same people over time found the decline accelerates, from about 3% to 6% per decade in your twenties and thirties to more than 20% per decade after seventy (Fleg et al. 2005, Circulation).

So the drop is gentle early and steep later, which makes what you build in midlife matter more than a flat average suggests.

How accurate is my watch's VO₂ max?

Good for spotting your direction of travel. Not good for knowing your actual number.

A meta-analysis of 14 validation studies covering 403 people found that exercise-based watch estimates were almost perfectly accurate on average, with the errors cancelling out to nearly nothing across the group. But for individuals the estimates ran from about 10 points below to 10 above the true value (Molina-Garcia et al. 2022). Across a crowd the errors cancel. For you personally, they don't.

A 2025 study tested Apple Watch Series 9 and Ultra 2 against laboratory measurement in 28 people and found an average error of about 13%, with the watch typically underestimating by roughly 6 mL/kg/min, and underestimating most in the fittest participants (Lambe et al. 2025, PLOS ONE). A broader review of 13 studies found Garmin devices generally landed within 10% but that accuracy degrades at the top end of fitness (Železnik Mežan et al. 2025).

The practical translation: if your watch says 42, your true value might reasonably be anywhere from the low thirties to the low fifties. But if your watch said 38 last spring and says 42 now, that change is meaningful, because the same device is making the same kind of error each time.

This is precisely why we compare you to you. A consistent measurement error is nearly harmless when you're reading a trend and actively misleading when you're reading a single number.

Can I actually improve it?

Yes, and more than most people expect.

A meta-analysis of training trials in healthy young-to-middle-aged adults found interval training improved VO₂ max by about 4.9 mL/kg/min on average, and steady moderate training by about 1.9 (Milanović et al. 2015). For scale, that is a meaningful shift against the age table above, though what a decade is "worth" depends heavily on your age given how the decline accelerates.

The important caveat is how unevenly that's distributed. In a study of 481 previously sedentary adults doing 20 weeks of supervised training, the average gain was solid, but individuals ranged from essentially no improvement to gains of more than 1,000 mL per minute (Bouchard et al. 1999). About 47% of the variation in how much people improved ran in families.

Read that carefully, because it's easy to garble. What's substantially inherited is how much you respond to training, not whether training works. Nearly everyone improves. How fast is partly luck of the draw, which is an argument for measuring against your own past self rather than anyone else.

Does fitness help even if my weight doesn't change?

The evidence says yes, and this is one of the more liberating findings in the field.

In a study of 9,777 men tested twice about five years apart, those who moved from unfit to fit had 44% lower all-cause mortality than those who stayed unfit. Every extra minute of treadmill time was associated with about 8% lower risk (Blair et al. 1995, JAMA).

A later meta-analysis of ten prospective studies found unfit people had roughly twice the mortality risk regardless of BMI, and that people who were overweight but fit had risk similar to normal-weight fit people (Barry et al. 2014).

So fitness appears to be worth pursuing on its own terms, whatever the scale does. If the scale is your current focus, reading weight trends without the noise is worth a look too.

How do you actually raise it?

Unglamorously, and mostly through volume.

• Build the aerobic base first. More total easy movement is the foundation, and the simplest entry point is your daily step count.

• Add some hard efforts once the base is there. Intervals produced the larger gains in the trials, though they weren't the only thing that worked.

• Give it months. The training studies run 12 to 20 weeks. VO₂ max moves slowly, which is exactly why a single reading isn't worth reacting to.

• Judge on the trend. Compare this quarter with last quarter, not this week with last week, for the same reason a single HRV night tells you so little.

We go into the training specifics, and how unevenly people respond to identical programmes, in how to improve your VO2 max.

Frequently asked questions

Is my watch's VO₂ max accurate?

Directionally, yes; precisely, no. Pooled across validation studies, watch estimates average out close to correct but carry limits of agreement of roughly ±10 mL/kg/min for individuals (Molina-Garcia et al. 2022). Apple Watch testing found around 13% average error with a tendency to underestimate (Lambe et al. 2025). Use it to track your own change over months.

What is a good VO₂ max?

It depends heavily on age and sex. Median values run about 48 for men and 38 for women in their twenties, falling to about 24 and 18 by the seventies (Kaminsky et al. 2015). Given the size of measurement error on a wrist device, where you sit relative to your own past matters more than where you sit on the table.

How quickly can I improve my VO₂ max?

Training trials in healthy young-to-middle-aged adults typically run 12 to 20 weeks and found average gains of roughly 4.9 mL/kg/min from interval training and 1.9 from steady moderate training (Milanović et al. 2015). Individual results vary widely, with roughly 47% of the variation in training response being familial (Bouchard et al. 1999).

Is VO₂ max mostly genetic?

Partly, but not in the way people usually mean. Your response to training is about half familial. That's different from saying fitness is fixed: in the 20-week study nearly everyone improved, just at different rates.

Why did my VO₂ max drop after a break?

Fitness is reversible, and estimates also drift with the data the watch has to work with. A drop after weeks off is plausible; a drop after a few days more likely reflects the estimate itself. Either way the trend across months is the signal.

Knit shows patterns, not diagnoses. Talk to a clinician about readings that concern you.