VO2 max: what the number means, how it is measured, and how far to trust a watch

What is VO2 max?

VO2 max is the highest rate at which your body can take in and use oxygen while you exercise as hard as you can. It is written in millilitres of oxygen per kilogram of body weight per minute, shortened to mL/kg/min. Garmin, which sells running watches, calls it the defining measure of cardiorespiratory fitness and aerobic performance capacity.¹⁰ That is a company’s description, not a clinical definition, but it matches how the number is used in the research below.

Because the figure is divided by body weight, two people with the same oxygen use in litres per minute can have different VO2 max values. A heavier person scores lower. That is worth remembering when you compare yourself with a friend, or when your weight changes and your watch quietly updates your score.

VO2 max and METs

Exercise scientists also use the MET, short for metabolic equivalent. One MET is 3.5 mL of oxygen per kilogram of body weight per minute, so you divide a VO2 value in mL/kg/min by 3.5 to get METs.⁵ The unit is handy because physical-activity advice, exercise machines and many research papers are written in METs.

How is VO2 max measured? Lab test, field test and watch estimate

There are three broad routes, and they do not give equally trustworthy answers. The first is a laboratory test. The second is a timed field test. The third is the estimate that a watch or band produces from your heart rate and pace.

The laboratory test

The reference method uses a metabolic gas analyser in a controlled laboratory.⁸ The American FRIEND registry, which collected 22,379 tests from apparently healthy adults aged 20 to 89, used treadmills for 16,278 of them and exercise bikes for 6,101.³ A lab test pushes you to your limit, which is why you should speak to a GP first if you have a health problem or have not exercised for a while (see the last section).

The mile walk

A field test costs nothing but a measured mile and a stopwatch. The 1987 study that built the walking equation concluded that the one-mile walk provides a valid sub-maximal assessment of VO2 max.⁶ The equation uses your age, sex, body weight in pounds, your walking time, and your heart rate over the last quarter mile. The result comes out in litres per minute, and you divide by your weight in kilograms to get mL/kg/min.⁶

A mile is 1.609344 km. Walking it in 15 minutes means covering 1.609344 km in a quarter of an hour, which is 6.44 km/h, or about 4 miles per hour. That arithmetic only tells you the pace the test asks about. It is not a target.

A weight conversion you will need

The equation wants pounds, and British scales often show stones and pounds. One stone is 14 lb, so 12 st 8 lb is 12 x 14 + 8 = 176 lb. To get kilograms, multiply by 0.45359237: 176 lb comes to 79.83 kg, which is the figure you divide by at the end. The conversion factor is the standard definition of the pound and was not taken from the sources read for this page.

What do British figures for VO2 max look like?

People often ask for a table of VO2 max norms by decade of age. This page does not print one. The 2022 American reference-standards paper (FRIEND) is only readable here as an abstract, and the tables that carry the age bands were behind a paywall.³ The one openly readable FRIEND table in the research for this page covers heart patients, not healthy adults, so it would be the wrong yardstick. A table built from other sources would mean inventing cells.

What does exist is a British study. Researchers working with Nuffield Health looked at estimated cardiorespiratory fitness in 9,204 men and 2,687 women, 11,891 adults in all, who were apparently healthy.⁹ The median estimated peak VO2 was 36.9 mL/kg/min in men and 36.5 in women, and the median age of both groups was 48.⁹

Read the table with its limits in mind. Participants stopped when they reached about 85% of their age-predicted maximum heart rate, using the 220 minus age rule, so the VO2 values are estimates rather than direct measurements.⁹ Only about 22% of the sample were women, which the authors flag as a limitation.⁹ The centile curves fell after the age of 30 to 40.⁹

The gap between the sexes in this table is small: 36.9 minus 36.5 is 0.4 mL/kg/min in the medians. That should not be read as a finding that sex makes little difference. The women’s group is much smaller, and the American registry, built on measured tests, found values higher in men across all age groups.³

What is a good VO2 max for your age?

Without a sourced table by decade, the honest answer is a set of reference points rather than a verdict. The American registry reports that median VO2 max falls with age, and that on average the decline was 13.5% per decade, or 4.0 mL/kg/min, over six decades of treadmill data.³ Dividing 4.0 by 3.5 gives about 1.1 METs lost per decade on average. These are American figures, and an average decline says little about any one person.

A worked example in METs

Suppose a watch or a test reports 38 mL/kg/min. Dividing by 3.5 gives 38 / 3.5 = 10.86, so about 10.9 METs.⁵ A meta-analysis of 33 studies and 102,980 participants grouped fitness as low below 7.9 METs, intermediate from 7.9 to 10.8 METs, and high from 10.9 METs.⁴ Multiplying back by 3.5 puts those boundaries at 27.65, 37.8 and 38.15 mL/kg/min.

So 38 mL/kg/min sits right at the edge of the high band. Set against the British study, a man of 45 at 38 would be a little above the median of 37 and below the 75th centile of 44.⁹ Both comparisons are rough, because one uses measured data in a pooled analysis and the other uses estimates in a single cohort.

How accurate is a watch or band estimate?

Garmin says its watches estimate VO2 max during a normal run, with no special test.¹⁰ Independent studies are less flattering, though they test particular models and small groups.

A mean absolute percentage error of 15.79% means that, on average, the watch’s answer was about 16% away from the lab value in either direction. For someone whose lab value is 40 mL/kg/min, that is an average miss of roughly 6 units, close to the 7-unit gap between the 50th and 75th British centiles for a 45-year-old man (37 and 44).⁹ The same study reports averages of 45.88 and 41.37 mL/kg/min for its two sets of values, a gap of about 4.5.⁸

Use the trend from one watch worn the same way, not the absolute figure, and do not compare scores across brands. A watch can also drift when your weight or pace changes. For a plain benchmark, the timed mile walk above is cheaper and better documented.

Can you improve VO2 max? What the sources say

The sources read for this page do not measure how much any training programme raises VO2 max, so this page gives no promised gain. What they do give is the standard UK advice on physical activity, which is where most people start.

• The NHS says adults aged 19 to 64 should aim for at least 150 minutes of moderate-intensity activity a week, or 75 minutes of vigorous activity.¹
• Brisk walking counts as moderate activity. The NHS gives a simple test: if you can still talk but not sing, the effort is moderate.¹
• The UK Chief Medical Officers use the same talk test and add that intensity is relative to a person’s own fitness.² A pace that is easy for one person can be hard for another.

That second point is the useful one for anyone with a low starting score. Moderate for you is whatever lets you talk but not sing, not a pace set by a friend’s watch. If your score is low, the effort that counts as moderate is a lower pace than it would be for a fitter person.²

To follow progress, repeat the same test under the same conditions every few weeks: the same route, the same time of day, and the same kind of footwear. A change of one or two units from one test to the next is within the noise of most estimates.

Who should see a GP before a hard test?

A laboratory test takes you to exhaustion. The NHS advises speaking to your GP first if you have not exercised for some time or have a medical problem.¹ The same applies if you plan to push hard to get a field test result. A timed mile walk is sub-maximal, but start by checking with your GP if any of those points describe you.⁶

If you feel chest pain, dizziness or unusual breathlessness during any test, stop. For an emergency, call 999. For urgent advice that is not an emergency, call NHS 111.

A watch estimate does not need that caution in the same way, but it does not make a medical judgement either. A low score on a watch is not a diagnosis, and a high one is not clearance to ignore symptoms. If a number worries you, talk to a GP rather than tuning your training around it.

Frequently asked questions