VO₂ Max Testing vs. Apple Watch, Garmin and Other Wearables

Your watch does not measure VO₂ max; it estimates it. That estimate can be genuinely useful for tracking trends, but independent research shows individual-level error can be large even when the averages look good. Here is how the major devices produce their numbers and when a direct test is worth doing.

What is the difference between measured and estimated VO₂ max?

A direct VO₂ max test measures the oxygen you consume and the carbon dioxide you produce, breath by breath, while exercise intensity rises on a treadmill or cycle. The number comes from your respiratory gases. ^[3] A wearable never sees your breath. It estimates VO₂ max from proxy signals: how fast you are moving, how hard your heart is working, what it knows about your age, sex, and body, and a proprietary model connecting those inputs to aerobic capacity. For a full explanation of what VO₂ max actually measures, see the pillar article.

Both approaches produce a number in mL/kg/min, which makes them look interchangeable. They are not. One is a measurement with known testing conditions. The other is an inference whose quality depends on the algorithm, the inputs, and how closely you resemble the population the algorithm was built on.

How does the Apple Watch estimate VO₂ max?

According to Apple's support documentation, Apple Watch Series 3 or later estimates cardio fitness (its term for VO₂ max) using the heart and motion sensors during Outdoor Walk, Outdoor Run, or Hiking workouts recorded in the Workout app. The estimate also factors in your age, sex, weight, height, and medications that may affect heart rate. Apple states the supported estimate range is 14 to 65 mL/kg/min, that indoor workouts do not count toward the estimate, and that an initial estimate can require at least 24 hours of wearing the watch plus several qualifying outdoor workouts and passive measurements. ^[5]

Two practical implications follow from Apple's own description. First, the estimate is built from submaximal outdoor sessions, not a maximal graded test. Second, anything that distorts the relationship between your movement and your heart rate, such as hills, heat, caffeine, stress, or heart-rate-affecting medication, can move the estimate around independently of your actual aerobic capacity.

How do Garmin watches estimate VO₂ max?

Garmin devices use analytics developed by Firstbeat. Firstbeat's documentation describes estimating VO₂ max from the relationship between speed and heart rate during walking or running, with results expressed relative to age- and sex-matched peers; cycling estimates on compatible devices use power and heart rate. Firstbeat's own guidance flags reliability limits: an unusually elevated heart rate (hills, heavy load, heat, prior exercise) can push the estimate down, while a blunted heart-rate response (for example from beta-blocker medication) can push it up. ^[6]

That honesty from the manufacturer is useful. It tells you the estimate is only as good as the heart-rate-to-speed relationship on a given day, which is precisely the link direct gas measurement does not have to assume.

How accurate are wearable estimates, according to independent research?

The most comprehensive independent evaluation is the INTERLIVE Network's 2022 systematic review and meta-analysis, which pooled 14 wearable-validation studies. For exercise-based algorithms, the average bias was close to zero. That sounds excellent until you look at the individual limits of agreement: approximately -9.92 to +9.74 mL/kg/min. For a single person, the watch could read 10 points high or 10 points low and still be within the study's expected agreement range. Algorithms built on resting information performed worse, overestimating on average with even wider limits of agreement. ^[1]

Accuracy also depends on the device and who is wearing it. A small 2025 validation study of 35 endurance athletes found a Garmin Forerunner 245 underestimated VO₂ max overall, with the largest errors in the most highly trained subgroup, exactly the population that watches small changes most closely. ^[2] That single study should not be generalized to every Garmin model or every athlete, but it illustrates why device-specific, population-specific validation matters.

Where are wearable estimates genuinely useful?

• Trend tracking on one device. Worn consistently, the same watch applies the same algorithm to the same person, so month-to-month direction can be informative even if the absolute value is off.
• A prompt to look closer. A very low estimate can be a reasonable prompt to look more closely at your activity and health, with appropriate professional input, including the relationship between fitness and long-term health.
• Convenience. The estimate arrives free from your daily walk or run, with no appointment and no maximal effort.

Where does direct testing provide different information?

• An accurate absolute value measured under controlled, recorded conditions, not an inference from heart rate and pace.
• Interpretation against reference percentiles matched to age, sex, and modality, so the number means something specific.
• Additional measured physiology: ventilatory thresholds, heart-rate response, and breathing patterns that algorithms do not estimate.
• A defensible baseline for retesting, where a real change can be distinguished from algorithm noise.

So which should you use?

For most people, the honest answer is both, used for different jobs. Wear your watch, watch the trend, and let it keep you honest about your training. When you want to know where you actually stand, when the number will inform a training plan or a health conversation, or when the trend on your wrist does not match how you feel, get it measured. One measured anchor makes every trend more meaningful.

Frequently asked questions

Can my Apple Watch measure VO₂ max?

No consumer watch measures VO₂ max directly. Apple Watch estimates it from heart and motion sensor data during Outdoor Walk, Outdoor Run, or Hiking workouts, combined with your age, sex, weight, height, and relevant medications. Independent research shows individual estimates can differ substantially from measured values.

Why did my watch VO₂ max drop even though I feel fitter?

Estimates respond to the heart-rate-to-speed relationship on your recorded workouts. Heat, hills, poor sleep, stress, caffeine, medication changes, or a different workout mix can move the estimate without any real change in your aerobic capacity. A single downward step on a watch is not proof of lost fitness.

Are some wearables more accurate than others?

Yes. Independent validation results differ by device and by the population tested. Exercise-based algorithms as a group perform better than resting-based ones, and at least one validation study found error grew in highly trained athletes. Check for peer-reviewed validation of your specific device.

Is a wearable estimate useless then?

No. A consistently used wearable is a reasonable trend tool, and trend direction is often what matters day to day. What it cannot reliably provide is a precise absolute value for an individual or a substitute for measured baseline testing.

How far off can a watch be?

In the INTERLIVE 2022 meta-analysis of exercise-based algorithms, individual limits of agreement were roughly -9.92 to +9.74 mL/kg/min. Resting-based algorithms performed worse. Your device could be closer than that, but the research shows you cannot assume it is.

When is a lab VO₂ max test worth it?

When you want an accurate baseline, when you are starting or changing a training program, when you want results interpreted against proper reference data, or when wearable numbers and felt performance disagree.