VO2 Max for Runners: What It Really Means
If you've spent any time in running circles, you've probably heard of VO2 max. It's the fitness metric that gets the most airtime. Most runners have a rough sense of what it means. High is good. Higher is better.
But VO2 max is actually just one piece of a bigger picture, and on its own it doesn't tell you nearly as much as most people think it does. There are a handful of numbers that sit alongside it, and together they give you a much clearer answer to the question that actually matters. Am I getting fitter?
Most runners never look at any of them. Here's what they are and why they're worth understanding.
VO2 max: the number everyone has and nobody fully understands
VO2 max is the maximum amount of oxygen your body can use during intense exercise, measured in milliliters of oxygen per kilogram of bodyweight per minute. Higher is better. It's widely considered the single best indicator of aerobic fitness and endurance performance.
If you have an Apple Watch or a Garmin, you already have a VO2 max estimate sitting in your Health app or watch dashboard right now. Most runners have seen it. Almost none of them know what to do with it.
Here's what it actually tells you. It's your aerobic ceiling. Every type of running you do, easy runs, threshold work, intervals, happens at some percentage of your VO2 max. When that ceiling goes up, everything underneath it gets easier. Your easy pace gets faster at the same effort. Your threshold pace improves. Your interval capacity increases. One number moving up lifts the entire system.
What counts as a good VO2 max for runners
This depends on age and sex, but as a rough reference for male runners: below 40 is below average for a recreational runner, 40 to 45 is average, 45 to 50 is above average, 50 to 55 is competitive amateur territory, and above 55 is approaching elite. For female runners, subtract roughly 5 to 8 from each range. Milo classifies these ranges automatically, so you can see at a glance where your current number sits.
These aren't hard boundaries. They're reference points. What matters more than where you are is whether the number is moving.
A VO2 max of 43 that was 40 three months ago is a much better sign than a VO2 max of 50 that's been flat for a year. The trajectory tells you more than the snapshot, which is why being able to see your trend over one month, three months, one year, or your full history is more useful than checking today's number in isolation.
Why your watch's VO2 max estimate is useful but imperfect
Your Apple Watch or Garmin estimates VO2 max from your pace and heart rate during outdoor runs. It's not a lab test. It infers your fitness from the relationship between how fast you're going and how hard your heart is working to get you there.
That makes it directionally useful but noisy day to day. A single reading can be thrown off by heat, humidity, hills, caffeine, or just a hard week. The number that matters is the trend over weeks and months, not any individual data point.
It also means the estimate improves when you give it cleaner data. Consistent effort on flat terrain in moderate weather gives your watch the clearest signal. Don't obsess over daily fluctuations. Watch the trendline.
What VO2 max actually means for your running training paces
VO2 max isn't just a fitness badge. It's the foundation your training paces are built on.
When your VO2 max improves, your training paces should change with it. An easy pace that was right at a VO2 max of 44 is too slow at 47. A threshold pace that was challenging at 44 might now be sustainable for longer. If your paces aren't updating as your fitness improves, you're effectively undertraining even if you're following a plan.
This is one of the most common mistakes intermediate runners make. They assess their fitness once, set their paces, and run those same numbers for months. Three months into a good training block, the paces no longer match where their fitness actually is.
VO2 max relative to weight: the running metric most people miss
VO2 max is expressed per kilogram of bodyweight, which means it's already a ratio. And that ratio can change two ways. Your aerobic capacity can improve, or your weight can change.
A runner with a VO2 max of 48 at 75kg has a different aerobic picture than the same runner at 70kg, even if the absolute oxygen consumption hasn't changed. Losing 5kg while maintaining the same training would improve VO2 max by roughly 3 to 4 points, which is the equivalent of weeks of hard interval training.
This isn't an argument for losing weight. It's an argument for understanding the full picture. If your VO2 max is stagnating but you've gained a few kilos, your aerobic engine might actually be improving and just being masked by the denominator. Seeing your VO2 max trend plotted alongside your weight over time gives you a much more honest view of what's actually happening with your fitness.
Lactate threshold: the other number that determines your race performance
VO2 max is your ceiling. Lactate threshold is how close to that ceiling you can actually sustain.
Your lactate threshold is the intensity at which lactate starts accumulating in your blood faster than your body can clear it. Below it you can run for a long time. Above it you're on borrowed time, minutes rather than hours.
For most trained runners, lactate threshold sits around 75 to 85 percent of VO2 max. The higher that percentage, the faster you can race at longer distances. A runner with a VO2 max of 50 who can sustain 85 percent of it will outperform a runner with a VO2 max of 55 who can only sustain 70 percent.
This is why VO2 max alone doesn't predict race performance perfectly. It sets the ceiling. Threshold, running economy, and fueling determine how close to that ceiling you can actually operate. And threshold improves primarily through threshold runs and tempo work, which is exactly why those sessions matter even though VO2 max intervals tend to get more attention.
Running economy: how to do more with less effort
Running economy is how much energy it costs you to run at a given pace. Two runners with the same VO2 max can have very different economy, with one burning less fuel at the same speed and therefore able to go further or faster before hitting their limits.
Economy improves through consistent running volume, speed work, and strength training. You can't easily measure it with a watch, but you can observe it indirectly. If your heart rate at a given pace drops over time, or if the same effort produces a faster pace, your economy is likely improving.
It doesn't show up on a single dashboard number, but it quietly determines how much of your VO2 max and threshold you can actually use on race day.
Building a complete running fitness picture
The runners who improve year over year aren't chasing one metric. They're building a picture. VO2 max trending up means the aerobic ceiling is rising. Threshold improving means they can sustain a higher percentage of that ceiling. Economy improving means they're using less energy at every pace. Weight stable or optimizing means the VO2 max ratio is telling the truth.
No single number tells the whole story. But together they tell you exactly what's working, what's stagnating, and where to focus next.
The problem is that most runners don't have an easy way to see this picture. The data exists, scattered across your watch, your Health app, your scale, your training log, but assembling it into something coherent takes effort most people won't sustain.
Milo puts it in one place. You can see your VO2 max trend over one month, three months, one year, or your full history, plotted alongside your weight so the relationship between the two is always visible. Your classification updates automatically as your fitness changes. And the training paces derived from your VO2 max update with it, so you're always training at the right intensity for where your fitness actually is right now, not where it was three months ago when you first set up your plan.
Your VO2 max from three months ago isn't your VO2 max today. Your training paces shouldn't be either.