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The more you work out, the fitter you’ll get. Your VO2max, sometimes described on wearables as a "cardio fitness" score, is a number that tends to be higher among people who are more cardiovascularly fit. But it’s not the only measure of fitness, and you should know what it really means.

What VO2max really is (as measured in a lab)

Smartwatches and fitness trackers don’t actually measure VO2max; they just estimate it. To actually find your VO2max, you’d need to go to an exercise lab and do a test on a treadmill or cycle while hooked up to equipment that measures your heart rate and the air you’re exhaling. I did exactly this kind of test to compare my real VO2max to the numbers reported by nine different devices. You can read about my experience, and why it matters, here.

VO2max, properly rendered VO2max, is the maximal volume (V) of oxygen (O2) your body can use per minute during exercise, scaled to your body size because larger people will naturally use more oxygen. This is considered a good measure of aerobic fitness because—think back to high school biology—your body uses oxygen and food to create a usable form of chemical energy called ATP. You then use that ATP to power the movements of your muscles. The amount of oxygen directly correlates with the amount of energy produced. Or, in simpler terms, the more work you do, the more oxygen you need.

So a person who is aerobically very fit—let’s say an elite marathon runner—can do more work (run faster) than somebody who isn’t. As a result, they’ll suck in a lot more oxygen per minute during their run than, say, I would.

What is a "good" VO2max?

A typical sorta-fit human being will have a VO2max somewhere in the 30s or 40s. If you exercise a lot, you can push that number higher. Elite cyclists, skiers, and ultrarunners may have numbers in the 70s or 80s, and a few people out there have scored in the 90s. I’d call myself a recreational athlete (I run a little) and my lab-tested VO2max was 43. Most of my fitness wearables reported numbers for me in the high 30’s or low 40’s—a good reminder that these estimates can be useful, but they aren’t necessarily correct.

Why is a high VO2max good?

To be able to use a large amount of oxygen, you need more than just good lungs. You also need a strong heart and healthy circulatory system to get the oxygen to your muscles. And you need muscles that are large and can do a lot of work per minute—which means that you’re strong and can do a lot of work athletically. VO2max is a way to summarize a lot of important things about cardio fitness into one simple number.

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But it’s important to know that this number isn’t everything about fitness, or even aerobic fitness. First, it doesn’t make much sense to compare your VO2max to other people’s, because it’s affected by factors like blood volume and muscle mass. Men tend to have a VO2max about ten points higher than similarly fit women, for example. There are also plenty of aspects of fitness that aren’t captured by VO2max. Compare two athletes with the same VO2max, and you may find that one can keep going in an endurance race longer than the other. Or one might be better at sprinting while another is better at medium-effort work. One person may be stronger than another, or have better technique, or be more resistant to injuries.

There is research correlating VO2max with longevity, but that doesn’t mean that raising your VO2max a few points will add years to your life. Fitness, in general, tends to correlate with longevity and with overall health. Scientists who want to study that relationship will usually pick an easy thing or two to measure, like VO2max or grip strength. But in real life, we don’t have to choose one number to maximize; we can work on multiple aspects of fitness. It’s better to be well-rounded than to hyperfocus on just one metric.

What do you think so far?

How can you measure your own VO2max?

The gold standard is a lab test. High-level athletes will sometimes take this test if they or their coach want to know their actual, definite VO2max number. But we can measure aerobic fitness in other ways, and there are workouts you can do to get a good estimate of VO2max. If you’re a runner, you can look up a recent race time on this chart to find your VDOT, which roughly correlates with your VO2max. (My best-ever mile time gets me a VDOT of 38, for example.)

To estimate your VO2max from a workout, you need something that will take you to your absolute maximum aerobic output. A race does this job nicely; so will a test like running as many laps of a track as you can in 12 minutes. I recall doing a test in a Phys Ed. class once where we had to step up and down from a high box as many times as possible. These tests aren’t pleasant, but their results can be a benchmark to tell you whether you’re improving your fitness over time.

How watches measure VO2max

Smartwatches and fitness trackers typically estimate VO2max by comparing your heart rate during exercise to a measure of how much work you’re doing during that exercise. Most often, this is through outdoor running or walking. The speed you run or walk is measured through GPS, and the device compares that to your heart rate. If you can run fast with a low heart rate, you must be pretty fit and your VO2max will be estimated as being pretty high. If your heart rate is high even for a slow running or walking pace, your VO2max is lower.

These estimates aren’t always accurate. If your max heart rate isn’t set properly in the device’s settings, if it’s hot out, if your running route has hills, or if you don’t happen to get out for outdoor runs or walks very often, your VO2max is more likely to be wrong. That said, your VO2max estimate doesn’t have to be exactly correct to be useful. If the number increases over time, you’re probably getting fitter. And if it decreases, you’re likely losing fitness.