What does VO₂ max show on a smartwatch and how accurate is it?

VO₂ max in smartwatches is an assessment of cardiorespiratory fitness: how efficiently the body can deliver oxygen to working muscles and use it during intense physical activity. A higher score, when correctly compared to people of the same age and gender, typically indicates greater aerobic fitness. However, the watch doesn't measure oxygen consumption directly; it calculates it based on heart rate, speed or intensity of exercise, movement, and user data. [1]

Therefore, a value of, for example, 42 ml/kg/min on a watch is not a laboratory result of VO₂ max. Modern devices are indeed capable of estimating the indicator quite well in some people, especially when using physical activity data, but individual error remains significant. A 2025 systematic review found 13 studies: in seven, wearable device assessments were considered valid or acceptable, but the results depended on the device, training status, and the method of assessment. [2]

In practice, VO₂ max on a watch is most useful as a long-term indicator of one's aerobic fitness: whether it increases after several months of training, remains stable, or gradually declines. A single, unexpectedly high or low reading is significantly less valuable. For sports diagnostics, medical assessment of exercise tolerance, or determining the cause of severe shortness of breath, cardiopulmonary exercise testing with respiratory gas analysis remains the standard. [3]

What does VO₂ max mean?

VO₂ max is the maximum amount of oxygen the body can use per minute during intense physical activity. On watches, this figure is typically expressed as milliliters of oxygen per kilogram of body weight per minute – ml/kg/min. Garmin, for example, calculates the displayed VO₂ max figure in this way. [4]

This isn't just a characteristic of the lungs. For oxygen to reach the working muscle, several systems must work together: the lungs ensure gas exchange, the heart pumps blood, the vessels deliver it to the tissues, hemoglobin transports oxygen, and muscle cells use it for aerobic energy production.

Therefore, the American Heart Association views cardiorespiratory fitness as an integral indicator of the body's ability to transport and utilize oxygen during prolonged exercise. It reflects the combined work of the cardiovascular, respiratory, and muscular systems. [5]

This is an important difference from blood oxygen saturation. SpO₂ measures the percentage of hemoglobin saturated with oxygen, while VO₂ max measures how much oxygen the entire body can deliver and utilize during exercise. A person can have a normal oxygen saturation of 98% and yet have low aerobic endurance.

What does the number 35, 40 or 50 ml/kg/min mean?

The number itself cannot be properly assessed without context.

VO₂ max varies significantly with age, gender, genetics, physical activity level, and training status. The American Heart Association therefore recommends interpreting cardiorespiratory fitness relative to the relevant population rather than applying a single, universal threshold to all individuals. [6]

For the same reason, apps typically compare results to age and gender references. Apple classifies aerobic fitness levels based on these two characteristics; Garmin also shows the user's position relative to their age and gender group. [7]

For example, the same value of 40 ml/kg/min may have a different position relative to the norm in a 25-year-old man, a 55-year-old man and a woman of the same age.

Therefore, the question "Is a VO₂ max of 40 good or bad?" doesn't have a universal answer. A more accurate question is: how does this result relate to age and gender, and how does it change in a particular person over time?

Why VO₂ max is related to health

Cardiorespiratory fitness is one of the most well-studied measures of physical fitness.

The American Heart Association's scientific statement identifies low cardiorespiratory fitness as an independent marker of increased risk of cardiovascular disease and overall mortality. Adding information about fitness may improve risk assessment beyond traditional factors. [8]

Higher cardiorespiratory fitness is also associated with better long-term outcomes in a number of other conditions. Recent observational data continue to support the association of higher fitness in midlife with longer life without multiple chronic diseases. However, these associations are population-level: an individual's score does not predict life expectancy. [9]

It's especially important to distinguish between true cardiorespiratory fitness and its assessment by watches. Strong predictive evidence primarily relates to physical fitness measured or validated by exercise testing. Consumer device data contains additional algorithmic error and should not be automatically used as a clinical risk indicator.

How VO₂ max is measured in the laboratory

The most accurate method is cardiopulmonary exercise testing.

A person performs gradually increasing exercise on a treadmill or bicycle ergometer. A special mask is used to analyze the volume of inhaled and exhaled air, oxygen consumption, and carbon dioxide output. Pulse rate, electrocardiogram, and other parameters are also assessed. [10]

This test allows one to directly determine gas exchange during exercise rather than relying on pulse rate as a proxy for oxygen consumption.

Cardiopulmonary testing is considered the standard method for assessing functional capacity during exercise. In clinical settings, it can also help determine why a person is experiencing exercise impairment—for example, whether the limitation is primarily cardiovascular or respiratory. [11]

Watches don't measure anything like that. There's no oxygen or carbon dioxide analyzer on the wrist. Therefore, VO₂ max is a mathematical prediction of the laboratory value.

How smartwatches calculate VO₂ max

The basic idea is simple: if a person has to maintain a certain speed at a relatively low heart rate, they are probably better aerobically prepared than a person who requires a significantly higher heart rate to do the same work.

The algorithm combines information about load and cardiovascular response. Manufacturers' specific formulas are proprietary and vary between devices.

Apple Watch uses heart rate and motion sensors while walking, running, and hiking outdoors. Age, gender, weight, height, and medication history that may affect heart rate are also factored into the calculation. A certain amount of accumulated data is required for an initial assessment. [12]

Garmin also uses heart rate in its current models; running requires outdoor training with a global positioning system, while cycling can use a compatible power meter. Garmin calculates separate VO₂ max values for running and cycling. [13]

This is why the watch is able to determine VO₂ max without forcing the user to undergo a true maximal test to complete fatigue.

Why is assessment during exercise usually more accurate than assessment at rest?

Because the algorithm receives much more information.

If the watch only knows the user's age, resting heart rate, and a few characteristics, it has to make a prediction based on rather indirect indicators.

While running, the device simultaneously monitors speed, pace changes, duration of exertion, and heart rate response. The correlation between these metrics and aerobic capacity is much more informative.

A meta-analysis of the INTERLIVE network included 14 validation studies. Algorithms using resting information overestimated VO₂ max by an average of approximately 2.17 ml/kg/min, with very wide individual margins of error. For exercise-based algorithms, the average bias was virtually zero—approximately −0.09 ml/kg/min. However, the margins of individual agreement still ranged from approximately −9.9 to +9.7 ml/kg/min. [14]

This is an important statistical nuance: a near-zero average error doesn't mean the watch is almost never wrong for a given user. Errors in different directions may simply cancel each other out.

How accurate is VO₂ max in modern watches?

On average, good enough to monitor fitness, but not good enough to consider every value a lab result.

A large 2024 review of systematic reviews reported that aerobic capacity estimation errors were approximately 15% for algorithms using resting data and approximately 10% when using exercise data. The authors also noted significant variability between devices and studies. [15]

A more recent systematic review from 2025 found 13 studies of consumer devices. Most of the data pertained to Garmin watches and algorithms that used outdoor running and heart rate data. Seven studies found VO₂ max estimates to be valid or acceptable compared to laboratory testing. The authors also noted a lack of long-term studies to understand the stability of the algorithms over months and years. [16]

Therefore, the correct formulation is not “the watch measures VO₂ max with an error of 10%,” but “in group studies, the error is often approximately in this range, but in an individual it can be significantly greater.”

What Apple Watch research has shown

The results of recent independent studies show significant individual error.

In a 2025 study, 30 participants first received an Apple Watch assessment and then completed a maximal indirect calorimetry test. The watch underestimated VO₂ max by an average of approximately 6.1 ml/kg/min. The average absolute percentage error was approximately 13.3%. [17]

A study published in 2026 involving the Apple Watch Series 10 included 40 healthy adults, equally divided between men and women. The device underestimated the laboratory value by an average of approximately 6.25 ml/kg/min, and the mean absolute percentage error was again approximately 13%. Individual limits of agreement were wide. [18]

This doesn't mean that the Apple Watch always underestimates the result by exactly six points. A universal "add 6" correction can't be applied: the direction and magnitude of the error varied among individual participants.

The results also do not allow the accuracy of one model to be automatically transferred to all generations of devices.

What Garmin's research shows

Garmin's results look better in some groups, but accuracy also depends on the user.

In a study of Garmin fēnix 6 in healthy active and sedentary adults, the mean absolute percentage error of VO₂ max was about 7%, and the researchers found the estimate to be reasonably close to the laboratory estimate.[19]

In a study of 35 endurance athletes, the Garmin Forerunner 245 had an average absolute percentage error of about 7-8% across two outdoor runs. However, for the most trained athletes, the watch systematically underestimated the value more significantly: the error was closer to 9-10%, while for moderately trained athletes it was less. [20]

This creates an interesting paradox: the algorithm may work very well for most amateurs, but worse for people with unusually high aerobic capacity.

This is one of the reasons why it is impossible to create a universal ranking of brands based on VO₂ max accuracy.

Can you trust your VO₂ max watch?

Yes - to track your own aerobic fitness, as long as you keep in mind the margin of error.

If the same watch, under similar conditions, showed approximately 37-39 ml/kg/min for months, and after several months of regular training, it consistently shows 43-45, this is much more interesting information than a change from 41 to 42 after one run.

A 2025 systematic review specifically points to the potential value of multiple repeated submaximal runs: repeated assessments may improve reliability compared to a single result. [21]

However, comparing the absolute performance of two different watches is more difficult. Some may consistently perform slightly higher, while others may perform slightly lower.

Therefore, after changing the device, it is reasonable to perceive the first weeks as the formation of a new baseline, and not to consider any change as a real improvement or deterioration in physical fitness.

Why is VO₂ max lower today than yesterday?

One small change does not mean that your fitness has worsened overnight.

The watch calculates VO₂ max based on the relationship between work performed and physiological response. If the optical heart rate sensor was working differently, the training conditions were different, or the nature of the load changed, the calculated value may also change.

Additionally, the manufacturer may use multiple workouts and passive measurements when updating the score. Apple explicitly states that the metric requires the accumulated data from multiple outdoor workouts and other measurements. [22]

Therefore, it is more reasonable to perceive small fluctuations as statistical noise around some true level.

A weekly or monthly trend is more informative.

Why medications can change the clock

Because algorithms often assume a certain relationship between exercise intensity and heart rate.

Some medications alter the heart's response to exercise. Apple explicitly takes medications that can affect heart rate into account when calculating aerobic fitness and warns that conditions or medications that limit heart rate may overestimate the estimated VO₂ max. [23]

This doesn't mean that physical fitness actually improved instantly. One of the algorithm's initial conditions changed.

Therefore, when starting or stopping a drug that significantly affects heart rhythm, comparing “before” and “after” values requires caution.

Why does body weight affect VO₂ max?

Because the most common form of the indicator is calculated per kilogram of body weight.

Let's assume a person is capable of using the same absolute amount of oxygen—for example, 3 liters per minute. If they weigh 100 kg, the relative result will be lower than if they weigh 80 kg.

Therefore, weight loss can increase VO₂ max in ml/kg/min even in situations where the absolute capacity of the heart and muscles to utilize oxygen has changed less. Similarly, weight gain can decrease the relative value. Scaling oxygen consumption by body weight itself is known to be a source of some limitations when comparing people of different body types. [24]

This doesn't make the metric invalid: relative oxygen consumption is indeed important when walking and running because a person has to move their own mass.

But this is precisely why a change in VO₂ max on the watch after significant weight loss cannot be automatically interpreted as an equivalent improvement in heart function.

Why does age affect the indicator?

With age, the average aerobic capacity of the population decreases, so the result should be compared with age-specific reference values.

Systematic studies confirm a strong dependence of maximum and peak oxygen consumption on age, as well as differences between men and women. [25]

Therefore, a value that is classified as low in a young athlete may be quite good in an older person.

This is why Apple and Garmin use age categories instead of one threshold for all users. [26]

What does low VO₂ max mean?

First of all, it is about low cardiorespiratory fitness relative to the corresponding group, and not about a specific disease.

Fitness levels are influenced by age, gender, genetics, frequency of physical activity, and a variety of physiological factors. Low cardiorespiratory fitness is indeed associated with increased long-term risk at the population level, but it does not in itself indicate why it is low. [27]

In a sedentary, healthy person, the cause may be simple detraining. In other situations, decreased exercise capacity may be due to cardiovascular, respiratory, muscular, or other limitations.

The watch is not capable of making such a distinction.

This is where the fundamental difference between the fitness function of a watch and a medical exercise test appears: cardiopulmonary testing simultaneously evaluates oxygen consumption, ventilation, carbon dioxide production, pulse, electrocardiography, blood pressure and symptoms and therefore helps to investigate the mechanism of reduced exercise tolerance. [28]

Can low VO₂ max be a sign of heart disease?

In itself - no.

Low physical fitness is a factor associated with cardiovascular risk, but it is not a diagnostic test for coronary artery disease, heart failure, or any other specific disease.

It is especially important not to draw such a conclusion based on a single assessment of consumer watches.

If a low or falling value is accompanied by new symptoms—for example, a noticeably worsened tolerance to normal exercise, unusual shortness of breath, chest pain or discomfort, dizziness, or loss of consciousness during exercise—then the symptoms, not the device number, are the primary reason to contact a doctor.

When it is necessary to objectively assess exercise tolerance and determine its cause, cardiopulmonary exercise testing is a much more informative method. [29]

Can a high VO₂ max mean the heart is completely healthy?

No.

High cardiorespiratory fitness is a favorable physiological indicator and is associated with a lower long-term risk of many adverse outcomes. However, it does not preclude hypertension, arrhythmia, atherosclerosis, or other diseases.

Moreover, the calculated value of the hours cannot guarantee this.

VO₂ max should be viewed as a single body characteristic, not as an integrated “heart health score.”

Why VO₂ max may differ between runners and cyclists

Because the result partly depends on the type of load.

Garmin directly uses separate VO₂ max estimates for running and cycling. Running estimates are based on heart rate and outdoor running speed, while cycling estimates can use power. [30]

Even when measured in a laboratory, the results obtained on a treadmill do not necessarily fully correspond to those obtained on a bicycle ergometer. Researchers who tested the Apple Watch specifically note that comparing results from different types of exercise can distort conclusions about the device's accuracy. [31]

Therefore, it is better to compare the same conditions: running with running, cycling with cycling.

Why does the watch sometimes not update VO₂ max at all?

Because the device must receive training that meets the requirements of its algorithm.

Apple Watch uses outdoor walking, running, and hiking for this feature. Indoor workouts and some other activities are not used to calculate your aerobic fitness score. [32]

Garmin's requirements vary by model. For example, the current Forerunner requires an outdoor run with a global positioning system and a certain intensity level for running evaluation; compatible power data is required for cycling evaluation. [33]

Therefore, after a heavy strength training or swimming, VO₂ max may not change at all, even though the load was high.

This doesn't mean the workout "doesn't improve your fitness"; it just doesn't meet the algorithm's requirements.

What is more useful: an absolute number or a trend?

For a watch user, it's usually a trend.

Situation	How to interpret
42 today and 41 yesterday	Most likely, practically the same thing
42 instead of the usual 43-44 after one workout	Don't jump to conclusions
Gradual increase from 35 to 40 in a few months	Aerobic fitness probably did improve.
A sharp jump after the clock change	It may be a difference in algorithms
Growth after significant weight loss	Part of the change may be due to the calculation per kg
Steady decline for several months	It matters more than one low number
Low score without complaints	Assess the trend and physical activity
Low score plus significant deterioration in exercise tolerance	Discuss with your doctor regardless of the time

Consumer algorithms are particularly attractive due to their ability to repeatedly assess physical fitness without the need for a comprehensive laboratory test. However, INTERLIVE emphasizes that individual error remains large enough for cautious use in sports and medicine. [34]

How big a change is considered real?

There is no universal number for all hours.

If the device has a potential inter-individual error of several ml/kg/min, a one-point change cannot be reliably distinguished from normal measurement variability.

For example, studies of the 2025-2026 Apple Watch found a mean absolute error of about 6-7 ml/kg/min compared to the laboratory. This doesn't mean the watch trend is completely useless—the systematic error of one device may be more stable across repeated measurements—but it does show why small fluctuations shouldn't be overemphasized. [35]

In practice, a consistent change over many comparable workouts is more credible, especially if it is consistent with changes in pace, heart rate, and subjective exercise tolerance.

Can VO₂ max be used to assess training progress?

Yes, this is one of the most sensible uses of the function.

If, at the same or faster pace, heart rate gradually becomes lower and estimated VO₂ max increases, this set of data is consistent with improving aerobic fitness.

Garmin research has shown that estimates obtained from multiple submaximal runs can correspond reasonably well to laboratory VO₂ max in some groups. A 2025 systematic review also notes that repeated runs may increase the usefulness of the estimate. [36]

But for an athlete who needs to know whether his true VO₂ max is 58 or 63 ml/kg/min, the watch may no longer be accurate enough.

This is especially true for well-trained athletes, for whom even a small mistake matters. [37]

Is it possible to improve VO₂ max?

Yes, aerobic fitness is a trainable characteristic.

Regular physical activity, especially endurance exercise, can improve cardiorespiratory fitness. The American Heart Association considers physical activity to be one of the major modifiable factors influencing this metric. [38]

The rate of change varies greatly between individuals, influenced by initial fitness, training volume and intensity, age, and individual physiological response.

Therefore, it's perfectly normal for the watch to show no noticeable change after just a few workouts. Real adaptations are measured in weeks and months, not days.

Conversely, a training program should not be built solely to increase VO₂ max numbers in the app.

What's more important: VO₂ max or resting heart rate?

These indicators answer different questions.

Resting heart rate describes the heart rate in a given physiological state. VO₂ max characterizes the body's maximum aerobic capacity.

A trained person often has both a relatively low resting heart rate and a high VO₂ max, but there is no direct correlation between “the lower the heart rate, the higher the VO₂ max.”

This is why algorithms use not just heart rate alone, but the ratio of heart rate to work performed – for example, running speed.

What's more important: VO₂ max or heart rate variability?

They are not interchangeable either.

Heart rate variability characterizes changes in the time intervals between successive contractions of the heart and is associated with autonomic regulation.

VO₂ max describes aerobic performance.

The first indicator can fluctuate significantly from day to day, while true aerobic capacity changes much more slowly.

Therefore, increasing heart rate variability does not automatically mean increasing VO₂ max and vice versa.

What is often misunderstood

"The watch measures how much oxygen I inhaled." No. It doesn't measure oxygen consumption; VO₂ max is predicted by an algorithm. [39]

"It's the same as saturation." No. Saturation describes the saturation of hemoglobin with oxygen, while VO₂ max measures the body's ability to utilize oxygen during exercise.

"A VO₂ max of 50 means I have a healthy heart." No. Good aerobic fitness is a positive sign, but it doesn't rule out cardiovascular disease.

"If the reading drops by two points, I'm out of shape." Not necessarily. The individual error of the devices is significantly greater than small daily changes. [40]

"All brands should have the same number." No. They use different sensors, algorithms, and criteria to calculate the score.

"The higher, the better." Higher cardiorespiratory fitness is generally beneficial, but indefinitely increasing it is not a medical goal in itself.

"Watches can replace laboratory VO₂ max." For routine monitoring, they can sometimes be a convenient alternative, but when an accurate, individualized measurement is needed, laboratory cardiopulmonary testing remains preferred. [41]

How to Use VO₂ Max Wisely

The most useful strategy is to use the same device, similar types of training, and a sufficiently long observation period.

Please ensure your age, gender, height, and weight are entered correctly in your profile. For Apple Watch, these parameters are included directly in your aerobic fitness assessment. [42]

Focus not so much on a single number as on a weekly and monthly chart. If the indicator gradually increases along with improvements in pace and exercise tolerance, this is much more convincing than a single successful measurement.

After changing your weight, medication, watch, or app algorithm, the new reading is not necessarily directly comparable to the old one.

If your goal is a clinical assessment of unexplained dyspnea, determination of true maximum physical capacity, or accurate sports diagnostics, another level of measurement is required: cardiopulmonary exercise testing. [43]

Key points from experts

Robert Ross, PhD, is a professor of exercise physiology in the School of Kinesiology and Health Studies at Queen's University and a member of the Division of Endocrinology and Metabolism in the School of Medicine. He led the writing group for the American Heart Association's scientific statement on cardiorespiratory fitness. [44]

In the statement, the working group considers cardiorespiratory fitness as an integral characteristic of physiological health and an independent predictor of long-term risk. Clinical value, however, is attributed specifically to physical fitness as a physiological indicator, and not automatically to any assessment obtained by a consumer device. [45]

Pablo Molina-García is a lecturer at the University of Granada and a researcher at the Institute of Sport and Health. He is the first author of a systematic review and meta-analysis of the INTERLIVE network on the accuracy of VO₂ max estimation by consumer wearable devices. [46]

The INTERLIVE research team concluded that algorithms based on physical activity data are significantly more promising than calculations based on resting indicators. However, individual error is still too great to rely unconditionally on consumer watches as a substitute for accurate measurements in sports or clinical practice. [47]

Frequently Asked Questions

Is VO₂ max on the watch the actual maximum oxygen consumption?

No, that's his estimate. True oxygen consumption is measured during a stress test with respiratory gas analysis. The watch predicts the result based on heart rate, speed or power, and other data. [48]

Is VO₂ max 40 good?

It depends on age and gender. Comparing 40 ml/kg/min to a single universal standard is incorrect. That's why Apple and Garmin use age- and gender-specific categories. [49]

Why does my VO₂ max drop even though I'm exercising?

Minor fluctuations may reflect algorithmic errors and specific workouts. Look at the monthly trend. If a sustained decline is accompanied by a noticeable deterioration in exercise tolerance, the situation should be assessed more broadly than just the hourly rate.

Why did VO₂ max increase after losing weight?

The indicator is usually calculated per kilogram of body weight. Therefore, weight loss alone can increase the relative value, even if absolute aerobic capacity has changed less. [50]

Which is more accurate - Apple Watch or Garmin?

There's not enough data to name a universal winner. Accuracy varies by model, user, and conditions. In some studies, Garmin showed an error of about 7%, while recent studies of Apple Watch showed an average absolute error of about 13%. However, these were different samples and protocols, and it's impossible to directly rank brands based on these figures. [51]

Why does Garmin show a separate VO₂ max for cycling?

Because running and cycling use different types of load and different input data, Garmin calculates these metrics separately. [52]

Can VO₂ max improve with walking?

In an untrained individual, regular aerobic activity, including moderately vigorous walking, can improve cardiorespiratory fitness. The extent of the change depends on the initial level and intensity of the activity. [53]

Should you aim for the highest possible VO₂ max?

There's no need to turn this into a competition. For health, it's especially important to move from low to higher cardiorespiratory fitness; a person doesn't need to become a high-level athlete to reap the benefits. [54]

Can you compare VO₂ max with a friend?

This is only possible very roughly. A more accurate estimate would be to take into account age, gender, body weight, type of exercise, and measurement method.

How often does it make sense to check VO₂ max?

There's no need to evaluate each workout individually. For most users, a weekly or monthly trend is more useful.

Do I need a lab test if my watch shows low VO₂ max?

Not just because of a single number. But laboratory testing may be warranted if an accurate athletic assessment is needed or if unexplained exercise intolerance is present, especially in conjunction with clinical symptoms. [55]

Main

VO₂ max in a smartwatch measures the body's estimated aerobic capacity—how efficiently it can deliver and utilize oxygen during exercise. It's one of the most insightful fitness metrics on a watch, as cardiorespiratory fitness is closely linked to physical performance and long-term health. [56]

However, the watch does not measure oxygen metabolism. Current research shows that estimates can fairly well reflect real VO₂ max at the group level and for many individual users, especially when using training data, but individual error can be around 10% and sometimes significantly more. [57]

Therefore, the most useful question is not "Am I getting the right number 42 today?" but "How does my number change over several months with the same device and comparable training?" For an accurate individual assessment of aerobic capacity or medical diagnosis, cardiopulmonary exercise testing remains the standard. [58]