How Does the Garmin VO2 Max Estimate Work? What You Need to Know

Have you ever wondered how the Garmin VO2 max estimate on your watch works?

VO2 max is one measurement of how fit you are and of your potential as an endurance athlete. Unfortunately, actually measuring your VO2 max is a complicated endeavor that includes a potentially costly trip to a sports clinic.

But if you have a Garmin running watch, you don’t need to make that trip. Your watch provides an estimate of your VO2 max.

Or at least that’s what it says.

Today, I want to explain what VO2 max is, how the Garmin’s algorithm works, and everything you need to know about the Garmin VO2 max estimate on your running watch.

What Is VO2 Max?

First, let me explain what VO2 max actually is. Without knowing that, it’s hard to explain the effectiveness and usefulness of Garmin’s estimate.

The “V” in VO2 max stands for volume, and the “O2” stands for oxygen. Simply put, VO2 max is the maximum amount of oxygen your body can use during an endurance activity like running.

When your body is engaged in any extended endurance activity – like running, swimming, cycling, or rowing – it does so aerobically. Your muscles need oxygen to create energy and keep you moving.

For a variety of reasons, there are limits to the amount of oxygen that your body can take in and use.

For one, there’s a limit to how much air – and oxygen – you can actually breath in. From there, only so much oxygen can be absorbed from your lungs into your blood. There’s also a limit to how much blood your body can pump through your veins. And, as the blood passes through the body, only so much oxygen move from your blood into the exercising muscles.

This is somewhat of a simplification – but the more fit you are, the more oxygen you can use in exercise. This is represented in a measurement of your VO2 max. If you had it lab tested, it would literally be a measurement of the maximum volume of oxygen, in milliliters, that your body can utilize, per kilogram of body weight and minute of exercise (ml / kg * min).

How Does Garmin’s Algorithm Estimate VO2 Max?

Getting a physical measurement of your VO2 max is a simple process. But it’s not one that your watch can do.

You need to go to an exercise lab that can measure the amount of oxygen that your body uses and run on a treadmill. It’s simple, but for most people it’s not accessible. You need to either be part of a research study or pay a lab to test you.

The good news is that there are ways to estimate your VO2 max without actually performing a physical test. One simple estimate is based off your maximum heart rate and your resting heart rate. Another is based of how far you can run in 12 minutes (a Cooper test).

Garmin has partnered with Firstbeat Analytics to utilize a more complex algorithm to provide an estimate of your VO2 max. The patent describing this algorithm is available online here.

Here’s, basically, how it works.

First, the algorithm assumes that you use a certain amount of oxygen while running. It uses a constant of 3.5 and your speed to estimate the volume of oxygen you are using at any given moment.

Next, the algorithm needs to know how much you weigh and what your maximum heart rate is. Weight is important since VO2 max is relative to the amount of weight moved. Maximum heart rate is important because it allows for an estimate of how intense an activity is.

Then, the algorithm identifies segments of an activity where you are running a relatively constant speed with a relatively constant heart rate. It chooses the most reliable segments and maps these to various heart rate zones.

Finally, the algorithm extrapolates what your speed would be at the moment of maximal oxygen uptake – and from that it can estimate your VO2 max.

Is the Garmin VO2 Max Estimate Accurate?

I wouldn’t blame you if you were skeptical that a watch can use an algorithm to estimate your VO2 max. After all, how can it actually know how much oxygen you are using?

Garmin watches and other devices using Firstbeat’s algorithm have been put to the test. The evidence suggests they aren’t perfect, but they are better than simple formulas.

Medicine & Science in Sports & Exercise

One group of researchers (Snyder, Willoughby, and Smith; 2017) tested the predictions made by a Garmin 230 and 235. Their results were published in Medicine & Science in Sports & Exercise as part of a conference.

They conducted a simple experiment using a relatively small sample – 18 women and 24 men. Each participant went for a run with the watches, and they later performed a traditional treadmill VO2 max test. The results for the women were relatively close (-1 +/- 4 ml/kg/min), and the results for the men were a little further off (-2 +/- 4 ml/kg/min). In the ballpark, but certainly not spot on.

Another group of researchers (Jolley, Carrier, Standifirid, and Creer; 2019) tested the predictions of a Garmin fenix 3. It was also published in Medicine & Science in Sports & Exercise as part of a conference.

Their sample was smaller (4 men, 2 women). But the sample was identified as recreational runners, and the methodology was marginally better. They used a heart rate strap to collect heart rate data, and their running protocol was to maintain an average heart rate of 70% for 15 minutes. The results showed no statistical difference between the Garmin VO2 max estimate and the results of an actual VO2 max test. However, the extremely small sample size is one reason to be skeptical.

International Journal of Environmental Research and Public Health

A third study (Passler, Bohrer, Blochinger, and Senner; 2019) tested the Garmin 920XT. It was published in the International Journal of Environmental Research and Public Health. Unlike the previous two studies, there is a complete article available instead of just a synopsis from a conference.

Their study included 24 people (13 men, 11 women), although it does not seem that they were runners. When they describe the Forerunner 920, they mention a chest strap, but from their methodology it’s unclear if they actually used one. The participants had to complete a “field-endurance -run at a self-chosen comfortable pace and for a duration of at least 10 minutes.” The results showed an average difference of -2 only ml/kg/min, but the overall results varied from -10.7 to +6.5. Again, in the ballpark. But certainly not 100% valid and reliable.

One the one hand, none of these studies had a particularly rigorous process. I think if there was a more well thought out protocol for the actual running, the results may be more reliable. I suppose this also reflects the way the watches are actually put to use. But I’ll save my thoughts on the state of exercise science for another day.

I think the bottom line here is that Garmin’s VO2 max algorithm has some validity, and it will give you a rough estimate of your VO2 max. But it’s far from perfect, and if you want to know your actual VO2 max, you’ll have to a lab to get tested.

What Are the Limitations of Garmins VO2 Max Algorithm?

There are a number of reasons why the estimated VO2 max might differ from the actual. Here’s a quick run down on a few.

Garbage In, Garbage Out

First, all algorithms rely on good data. Garbage in, garbage out. If you don’t have accurate heart rate data, an algorithm that is based on heart rate data will give you bad results. Using a heart rate strap, like the Garmin HRM Dual, would likely increase the validity and reliability of the algorithm. You can read more here about some of the problems associated with wrist based heart rate monitors.

In the same vein, the algorithm relies on an accurate estimate of a person’s heart rate zones and their maximum heart rate. If any of those are off, its validity will also be off.

Not All Runs Are Equally Useful

Second, consider the actual run itself. The algorithm is extrapolating from the pace and heart rate at various zones. If you spend your entire run at a conversational pace, it may be difficult to make that extrapolation. Likewise, if your pace varies constantly throughout your run, the data may be skewed. You’d expect the most validity from a run that included several continuous minutes at various intensities, including a bout at or near your 3k or 5k pace.

Environmental Factors Impacting Heart Rate

Third, there’s the issue of the many things that can influence your heart rate and energy expenditure on a run. Weather is one. Running on a hot day, especially when you aren’t acclimated, will artificially increase your heart rate. On a longer run, dehydration and fatigue may skew the results. And if you aren’t running on pavement or concrete, you might be getting less energy return from the ground – and essentially working harder at a given pace. In theory, the algorithm could take into account weather, elevation, and ground surface. But in practice, your Garmin watch doesn’t do so (or doesn’t do so effectively).

Don’t Forget About Running Economy

Finally, the algorithm is based on an assumption about effort and heart rate. It assumes that everyone uses the same amount of oxygen to run at a given pace, and that their effort level as measured through heart rate can be consistently matched against different paces. But different runners have different running economies, and the same runner may have a different running economy at different paces depending on their training. As a result, it’s likely that algorithm may be reliable in that it estimates the same VO2 max for the same person in multiple runs – but not valid because it over or underestimates the actual value.

So Should You Pay Attention to Your Garmin VO2 Max Estimate?

At the end of the day, Garmin’s VO2 max estimate is useful as long as you understand its limitations.

It will not – and cannot – give you a perfectly accurate and reliable measurement of your VO2 max. But, in ideal conditions, it can give you a relatively reliable measurement.

And if you are only concerned with measuring your progress against yourself, it doesn’t really matter whether it’s valid or not. If it underestimates your VO2 max by 5 ml/kg/min, but it does so every time it estimates your VO2 max, then you can still clearly see your progress over time.

Combine that with the understanding that individual runs may be impacted by environmental factors. At the beginning of summer, the heat may set your pace back as well as your VO2 max. And if you do a couple runs over gravel trails, you might seem slower and less fit.

But if you zoom out, and look at your VO2 max estimate across several months or years you can get an idea of how things are going. And that is how you should think about the number you see on your watch.