VO2 max on a smartwatch: what validation studies show about accuracy

Oct 23, 2026

Prepared by: Research Desk. Sources are linked for verification.

Many fitness watches now display a number called "VO2 max." It looks precise, often to one decimal place, and it can go up or down from week to week. A watch does not measure oxygen directly, though. It estimates the figure from heart rate, movement and personal details.

This explainer sets out what VO2 max is, why researchers care about it, and what published validation studies report when watch estimates are compared with laboratory measurements. It summarises research only and is not advice for any individual.

What VO2 max is, in plain English

VO2 max is the highest rate at which the body can take in and use oxygen during hard exercise. It is usually expressed as millilitres of oxygen per kilogram of body weight per minute. Researchers use it as the standard measure of cardiorespiratory fitness: how well the heart, lungs, blood vessels and muscles work together.

The reference method is a laboratory exercise test, often called cardiopulmonary exercise testing. A person exercises on a treadmill or bike while wearing a mask that measures the oxygen and carbon dioxide in each breath. The effort gets harder until the person reaches their limit. This takes specialist equipment and trained staff, which is why a wrist-based estimate is appealing.

Why researchers pay attention to fitness

Cardiorespiratory fitness is one of the measures most strongly linked with long-term survival in observational research. In a retrospective cohort study of more than 120,000 adults who completed exercise treadmill tests at Cleveland Clinic, higher measured fitness was associated with lower long-term mortality [1]. This association continued all the way up to the highest-fitness ("elite") group [1]. The study was observational, so it shows an association, not proof that fitness alone caused the difference.

This matters for healthspan, the years lived in good health. Researchers have noted that life expectancy has risen much faster than healthspan [2]. Fitness is one of several measurable factors studied in that gap. U.S. federal physical activity guidelines also describe aerobic activity as a core part of an active lifestyle for adults [3]. Our evidence-ranked healthspan research guide covers how fitness compares with other factors such as daily steps and muscle-strengthening activity.

Because fitness is so widely studied, people naturally want to track it. The key question is how close a watch's estimate gets to a laboratory test.

How a watch estimates VO2 max

Methods differ by brand and are often not fully published. Broadly, watches combine:

Heart rate from an optical sensor on the wrist (or a paired chest strap);
Speed or workload, usually from GPS during outdoor runs or walks, or from a power meter on a bike;
Personal details such as age, sex, height and weight;
Sometimes resting data, such as resting heart rate, without any exercise at all.

The estimate is only as good as these inputs. Validation research on wrist-worn devices has found that heart-rate accuracy can vary with the type and intensity of activity [7]. Energy-expenditure estimates have generally performed worse than heart rate [5][7]. Any error in the heart-rate signal can feed into the VO2 max estimate.

What the validation studies report

The big picture: a meta-analysis

The most useful single source is a systematic review with meta-analysis by the INTERLIVE network [4]. It pooled studies that compared consumer wearables' VO2 max estimates against laboratory-measured VO2 max. The authors reported that accuracy depended on how the estimate was produced. Estimates based on exercise data performed better than estimates based on resting data [4]. The paper also includes an expert statement. It recommends how future validation studies should be designed and reported, so results can be compared fairly across devices [4].

In plain terms, a watch that estimates VO2 max from a recorded run or walk is, on average, a better bet than one that estimates it from resting measures alone. Even so, individual readings can still miss the lab value.

Individual device studies

Smaller human validation studies add detail:

Wrist-worn activity trackers (2019). A study of several wrist-worn trackers compared their VO2 max estimates and energy-expenditure estimates with laboratory measurements. VO2 max estimates were reported as closer to laboratory values than energy-expenditure estimates, which showed larger errors [5].
A recent smartwatch model (2024). A validation study tested one widely sold smartwatch against laboratory VO2 max. It reported that the watch's estimates tended to fall below the measured values. The authors judged the overall error too large for the estimate to stand in for a laboratory test [6].
Heart rate inputs (2020). A validation study of wrist-worn wearables examined heart-rate accuracy while sitting and during light-to-vigorous activity. Energy-expenditure estimates were less accurate than heart rate [7].

These are single-device or small-sample studies. Results apply to the models and firmware tested and may not carry over to newer versions.

Tracking change over time

A single reading is one question. Tracking whether fitness is rising or falling is another. A study in NPJ Digital Medicine used wearable sensor data collected during everyday life, combined with machine-learning models, to predict cardiorespiratory fitness and changes in it over time in a large population cohort [8]. This research suggests that wearable data may capture trends, but it tested research-grade methods, not the built-in VO2 max feature of any consumer watch.

Specialist and sport settings

A case report and literature review on wearables for peak oxygen consumption in cardiology cautioned that consumer estimates are not yet a substitute for formal cardiopulmonary exercise testing where precise values are needed [9]. A 2026 systematic review of wearables in team sports examined how validity differs across devices and the metrics being measured [10]. Researchers have also explored smartwatch-derived VO2 max as an input for predicting acute mountain sickness in a diagnostic accuracy study [11]. That is a reminder that these numbers are increasingly used in research, which makes knowing their error all the more important.

Why accuracy varies so much

Across these studies, a few recurring reasons explain why one person's watch reading may be close to a lab value while another's is not:

Resting versus exercise estimates. Exercise-based estimates have generally performed better [4].
Heart-rate signal quality. Wrist optical sensors can be less reliable during some activities [7]. Some watches can also pair with an external chest strap, a different way of capturing heart rate.
Fitness level and population. Estimation methods are developed and tested in particular groups of people, so accuracy may differ for people unlike those groups.
Device and software version. Results from one model or algorithm do not automatically apply to another [6][10].
Study design. Validation studies differ in methods, which is why the INTERLIVE statement calls for standardised approaches [4].

How to read the number sensibly

The research supports regarding a watch's VO2 max as an estimate with a margin of error, not as a laboratory measurement. Several research-based points follow:

Trends tend to be more informative than single readings, as long as the same device, settings and type of activity are used [8].
Exercise-based readings have generally shown better validity than resting-only readings [4].
Small changes may sit within the error range. A shift of a point or two may reflect measurement noise rather than real change.
A laboratory test remains the reference standard where an exact value matters [9].

Watch makers' marketing language around fitness and longevity is not always held to the standard of a peer-reviewed validation study. Our guide to how longevity products and claims are regulated explains how to read such claims critically. The FDA's consumer page on health fraud scams lists red flags such as one product claiming to do everything [12]. For how this site chooses sources and handles affiliate links, see About this site.

Choosing a fitness watch on neutral criteria

For anyone comparing watches mainly for VO2 max estimates, these criteria follow from the studies above:

Published validation. Look for independent, peer-reviewed comparisons with laboratory testing for the specific model, not just the brand [4][6].
How the estimate is produced. Check whether the watch uses outdoor runs or walks with GPS, cycling power, or resting data only. Exercise-based estimates have the stronger track record [4].
Heart-rate options. Since wrist heart-rate accuracy can vary with activity [7], note whether the watch can also pair with an external heart-rate strap.
Trend view and data export. A clear history chart and the ability to export raw data make long-term tracking more meaningful.
Price transparency. Note whether advanced fitness metrics need an ongoing subscription.
Data privacy. Review what is shared, with whom, and whether data can be deleted.
Battery life and comfort. A watch that is worn consistently produces more consistent data.

Tools for comparing fitness watches

Fitness watches differ widely in sensors, estimation methods and published validation. The checklist above is a practical way to compare models side by side before choosing one.

When reading product listings, check the model name and year against any validation study cited, since results for older models may not apply.

Frequently asked questions

How accurate is VO2 max on a smartwatch?

It depends on the device and how the estimate is produced. A meta-analysis of validation studies found that exercise-based estimates performed better than resting-based ones [4]. Single-device studies have still reported meaningful differences from laboratory values [6].

Is a smartwatch VO2 max the same as a lab test?

No. A laboratory test measures oxygen uptake directly from breathing gases during maximal exercise, while a watch estimates it from heart rate, movement and personal details. Researchers writing about cardiology settings have cautioned that wearable estimates are not yet a substitute for formal testing [9].

Is it worth tracking VO2 max trends on a watch?

Research using wearable data in everyday life suggests fitness changes over time can be modelled from sensor data [8]. Trends are generally more informative than one-off readings when the same device and activity type are used. Small week-to-week shifts may fall within measurement error.

Last updated: 19 October 2026.

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