VO2 max represents the maximum amount of oxygen your body can use during intense exercise. It's widely used to assess cardiorespiratory fitness.

But unlike a laboratory test, an Apple Watch doesn't measure the oxygen you breathe in or out. Instead, it estimates VO2 max by combining data from sensors with information about your body and physical activity.

That raises an interesting question: How can a device sitting on your wrist estimate something happening deep inside your cardiovascular and respiratory systems?

With Apple Watch Series 12 and Apple Watch Ultra 4 introducing newer health-sensing capabilities alongside Apple's new at-home VO2 max assessment experience, understanding that number requires looking at more than just the result on the screen.

What We'll Cover:

• What Exactly Is VO2 Max and Why Does It Matter?

• The Original VO2 Max Test Was Not Designed for Your Wrist

• So How Does a Watch Estimate Something It Can't Directly Measure?

• Meet the New Apple Watch VO2 Max Experience

• Apple Watch Series 12: Why Better Heart-Rate Data Matters

• Apple Watch Ultra 4: Why GPS Can Add Important Context

• What Happens When You Stop Moving but Your Workout Keeps Running?

• Why VO2 Max Isn't Just a Speed-versus-Heart-Rate Equation

• But How Accurate is the Apple Watch's VO2 Max Estimate?

• Wrapping Up

What Exactly Is VO2 Max and Why Does It Matter?

To decipher what your watch tells you, it's important to first understand the science of the metric. VO2 max essentially refers to the maximum volume of oxygen that can be used by the body while working at high intensities.

It's important for the body to continuously have oxygen available so that it can produce energy. Whether you're walking, jogging, cycling, or running, your body needs several biological systems to coordinate flawlessly.

The first step is for the lungs to bring oxygen to the body. After that, our heart pumps oxygen-rich blood very fast through the veins. The vital oxygen goes directly to the body muscles where it's used to produce the energy needed for the body's activity. The efficiency of the whole process is expressed by the VO2 max.

It's often presented using mL/kg/min, which just translates to milliliters of oxygen consumed per kilogram of body weight per minute. You can view VO2max as a rather crude estimate of how well your body is able to consume oxygen in situations of very high energy demand.

Due to the link between cardiorespiratory fitness and various health outcomes, VO2max is often used in exercise physiology and health literature.

The Original VO2 Max Test Was Not Designed for Your Wrist

The Lab Version

Traditionally, to determine exact VO2 max, physicians would apply a relatively strenuous method. For example, the test subject would run on the treadmill or perform strenuous exercise on the stationary bike. At the same time, they'd be wearing the mask, which would be tightly attached to a metabolic analyzer. As the exercise got more intense, the test subject would start breathing harder and harder.

At the same time, the subject would need to exert themself until they reached their peak level or total exhaustion. While exercising, the complex equipment would precisely measure the amount of oxygen that entered into their body and the quantity of carbon dioxide being released. This detailed measurement of their breathing is why lab testing is the traditional/preferred method of determining VO2 max.

But herein lies a huge practical issue: though the lab method is quite precise, it's not something you'd randomly do (or easily be able to do) after work on a Tuesday night, for example. This practical problem is partly why wearables have come into play.

So How Does a Watch Estimate Something It Can't Directly Measure?

Again, note that an Apple Watch doesn't measure your metabolism via a direct test, like measuring cardiorespiratory function using a lab breathing mask. The device calculates cardiorespiratory fitness by utilizing complicated mathematical equations involving your physiological and workout data.

Apple openly admits that cardiorespiratory fitness is measured by VO2 max and workouts. In calculating, Apple takes into account a number of parameters, including age, gender, weight, height, medications that might influence heart rate, and a great deal of sensor data.

In other words, the watch focuses on one basic principle: how hard your heart is working compared to the effort you're producing physically.

Let's take two people running at the same brisk pace. Person A has a rather low heart rate while keeping this pace, showing that their cardiovascular system works effectively. Person B has a significantly higher heart rate while maintaining the same pace.

Over time, it's possible for an advanced algorithm to figure out the differences in their cardiorespiratory fitness. The algorithm uses relationships between all those data points. And while the exact calculation method isn't public, the physiological concept behind it is pretty clear.

Meet the New Apple Watch VO2 Max Experience

The hardware updates released in late 2026 came with significant improvements in interacting with and interpreting these metrics.

In particular, the newly designed Health feature actively supports at-home measurements, giving users the option to take an accurate and guided test to evaluate their VO2 max. According to Apple, you can perform this evaluation using either the Apple Watch Series 12, Apple Watch Ultra 4, AirPods Pro, or any other compatible heart rate sensor device.

This Is More Than Just Looking at Your Workout History

Previously, only VO2 max estimation from certain background outdoor exercises was allowed, like outdoor walking, outdoor running, and hiking. In other words, the software would just passively monitor your activity while you were doing something in real life.

Now, the newly designed Health experience brings you closer to a more accurate clinical-like assessment of VO2 max. In contrast to passive measurement based on random monitoring by your Apple Watch, the current version of the application lets you conduct a guided at-home physical test to evaluate your VO2 max.

Apple Watch Series 12: Why Better Heart-Rate Data Matters

To ensure that these figures are accurate, the quality of the raw data should be high. The latest generation of the Apple Watch Series 12 is equipped with Apple's innovative Health Sensing System that provides heart rate and HRV (heart rate variability) monitoring at significantly higher frequencies. In fact, heart rate background measurements are made every 5 seconds. According to Apple, this advanced sensing system gives the most accurate heart-rate sensing ever seen on a wearable device.

Heart Rate Is One of the Most Important Pieces of the Puzzle

When you undergo any kind of strenuous exercise, your muscles need more energy than usual. This means your body needs more oxygen, so your heart tends to work harder and faster to respond to that.

But heart rate by itself doesn't just magically reveal your VO2 max. The device needs to analyze your heart rate along with the precise details of your physical activities, how intensely you exercise, your pace of running, and other health information. Then it can really help in estimating your cardiorespiratory fitness.

Bear in mind that the optical heart-rate monitor doesn't literally detect the oxygen flow in your muscles. Rather, it gives one of the key physiological inputs used by the complex estimation model to crunch its numbers.

Apple Watch Ultra 4: Why GPS Can Add Important Context

Although the Series 12 enhances biological sensing, the Apple Watch Ultra 4 prioritizes high spatial precision. In fact, Apple advertises that the Ultra 4 features very precise GPS functions that are specifically designed for tough sporting and outdoor pursuits. Indeed, VO2 max is listed amongst its premium running exercise metrics.

Movement Data Gives Heart Rate a Story

The fact of the matter is that your heart rate alone doesn't indicate what you're doing physically. Your heart rate may be 150 beats per minute because you're jogging uphill, sprinting along a flat surface, exercising in extreme summer temperatures, climbing several flights of stairs, or even engaging in a completely different physiological response altogether such as being stressed out.

Movement and location-specific data about workouts can help fill in this information gap. This is because the watch can match up your heart rate with all sorts of analysis related to your pace, total distance traveled, drastic change in movement, workout time, and, more importantly, your geographical location.

What Happens When You Stop Moving but Your Workout Keeps Running?

When your run comes to a sudden halt, your legs stop moving, but your heart hasn't received the memo yet. The watch on your wrist continues actively collecting workout information. The internal motion sensors can detect major, sudden changes in your movement, while GPS-equipped models effortlessly record the immediate drop in your pace and your stationary location during the outdoor activity. Meanwhile, your heart rate may easily remain elevated even after your physical movement slows down or stops entirely.

Rapid changes in movement and cardiovascular effort can create incredibly complicated physiological data. A person's running pace may suddenly drop to zero while their heart rate remains high simply because the human cardiovascular system doesn't instantly return to its resting baseline levels.

This means that estimation algorithms continuously need to interpret those changing signals and accurately determine which specific parts of the recorded activity actually provide useful, stable information about your cardiorespiratory fitness.

Because Apple doesn't publicly provide the complete proprietary algorithm, we can't definitively claim that an unexpected pause automatically produces a lower VO2 max score. The software is likely designed to filter out these brief, noisy anomalies, but it highlights exactly why interpreting wearable data is such a difficult engineering challenge.

Why VO2 Max Isn't Just a Speed-versus-Heart-Rate Equation

While comparing your pace with your heart rate would help you understand the idea generally, the real science behind VO2 max estimation is much more complex.

The way that Apple employs in its VO2 max estimation process is highly dependent on several individual and environmental factors that could have a huge impact on your heart rate response. Some of those factors include your age, your biological gender, your body weight, your actual height, the heart rate-affecting medicines you are taking daily, the particular type of exercise, the level of your effort and the quality of the sensor readings.

The official support documentation provided by Apple states that your watch is capable of estimating your VO2 max in a range of 14-65 mL/kg/min, ensuring it covers a broad spectrum of human fitness levels.

But How Accurate is the Apple Watch's VO2 Max Estimate?

This is where the marketing promises meet peer-reviewed scientific reality. The research regarding wearable accuracy is nuanced and we can't simply declare that the watch accurately measures VO2 max without looking closely at the data.

What Scientific Studies Say About Apple Watch VO2 Max

A 2025 validation study published in PLOS ONE compared Apple Watch VO2 max estimates directly with indirect calorimetry (the strict lab standard). The researchers found that the Apple Watch actually underestimated VO2 max on average and reported a mean absolute percentage error of approximately 13%.

The study thoughtfully concluded that these estimates still require refinement before true clinical implementation, although the watch may remain practically useful as a more accessible alternative to conventional lab testing.

In other words, the watch was useful for producing a convenient estimate, but individual results could differ meaningfully from the exact value measured by a doctor in a lab.

Also, a 2024 validation study evaluating the older Apple Watch Series 7 also compared wearable estimates with laboratory testing using a metabolic gas analyzer. The researchers found notable differences between the predicted and laboratory-measured values.

This doesn't mean the Apple Watch is useless. But again, it means that a we shouldn't treat a convenient wearable estimate as identical to a rigorous laboratory measurement.

A more recent 2026 study evaluating the Apple Watch Series 10 echoed these precise findings, noting that the watch underestimated VO2 max overall compared with indirect calorimetry and showed meaningful variability at the individual user level.

The authors suggested that wearable estimates may still hold great value in research or broad population-level contexts but require further longitudinal validation for individual measurement reliability.

Wrapping Up

Apple Watch Series 12 and Ultra 4 make VO2 max easier to assess by combining heart-rate, movement, and other health data to estimate cardiorespiratory fitness. While the new at-home assessment offers a more structured way to get that estimate, the watch still can't directly measure oxygen consumption like a lab test can.

The number, then, is best understood as an estimate, but an interesting one that shows how sensors, physiology, and algorithms can work together to reveal more about your fitness.

I know you must be thinking that doesn't make sense: two different people are never going to have the same heart rate numbers anyway. Different resting baseline, different fitness levels, different everything. And yes, that part has nothing to do with wearables at all. But it's almost a competition at this point.

The real question here arises when you take the comparison of the same heart rate metric with different wearables but on the same person. The heart rate reading will be different – again because it will depend upon the type of sensors used in each case, the speed of sampling, and the software algorithms of the sensors.

After you learn how they work internally, you'll finally be able to understand why the numbers are different.

Table of Contents

• They're All Measuring the Same Thing, Just Not in the Same Place

• Why Your Heart Rate Isn’t Constant

• Sampling Speed Changes What the Device Actually Catches

• The Algorithm Matters as Much as the Sensor

  • Skin Tone and Wrist Size Add Another Layer

• Activity Type Throws Everything Off Differently for Each Brand

• What This Means If You're Comparing Numbers With a Friend

They're All Measuring the Same Thing, Just Not in the Same Place

All of the wearables worn on your wrist or finger use the same technique known as photoplethysmography (PPG). An LED shines light into your skin while a sensor measures how much light bounces back with each heartbeat. The technology remains the same among all manufacturers, but the difference here lies in the place of measurements.

Finger-based wearables measure arteries located closer to the surface of the skin than the ones on your wrist. This gives finger-based devices like the Oura Ring a real advantage.

This advantage becomes clear even in the sleep stage, since the movements of your fingers are smaller than the movements of your wrist during sleep. Research done to test the accuracy of the heart rate reading by the Oura Ring has been able to show high consistency with the ECG measurements.

This is one reason Oura tends to perform well in nocturnal heart rate verification. A comparative study conducted in 2025 in the journal Physiological Reports pitted 5 devices (the Oura Ring Gen 3, Oura Ring Gen 4, Whoop 4.0, Polar Grit X Pro, and Garmin Fenix 6) against a medical-grade ECG for 536 nights of sleep. The Oura Ring 3 and 4 correlated the most accurately with ECGs, WHOOP correlated moderately, and Polar had the lowest accuracy.

Why Your Heart Rate Isn’t Constant

One thing you need to know is that your heart rate isn’t constant even at rest.

It varies all the time with your breathing movement or any other stress response, even when you're sitting completely still. 68 bpm can become 72 bpm after just a few seconds and nothing out of the ordinary happened.

It's enough for two wearables to have slightly different readings just because they were measuring at different times. Before sensor location, sampling rate, or any other algorithm comes into play, what's being measured is already constantly changing.

Sampling Speed Changes What the Device Actually Catches

Position of the sensor is important, but frequency of readings is much more important than you might think. The PPG sensor from WHOOP 5.0 and WHOOP MG records reading 26 times per second. This is considered to be quite a good rate of recording from a continuous wearable.

Other smart devices, on the contrary, work every few seconds and not continuously. As a result, if your Heart Rate suddenly spikes for any reason, like standing up too quickly or being startled, the device will miss that spike and fill in the gap to replace a missing value.

The Apple Watch also has a different mechanism: it has a high hardware sampling rate, but that doesn't mean that its heart rate sensor works continuously. In reality, it works periodically depending on your activity, and only works continuously while working out and for several minutes after the exercise.

None of this is necessarily better in an objective sense. The continuous heart rate frequency sampling is fantastic for capturing all those short-term spikes, but results in more raw data that needs to be filtered and sorted out by the algorithm.

Garmin takes a middle ground here, depending on the model of the watch. In general, most Garmin devices take continuous samples at low frequencies all day long until they detect the start of your exercise and increase sampling frequency automatically.

And there's another trade-off here: the device saves battery when you're working at your desk, but this also means that the device is essentially betting on its own movement detection being accurate before it decides that your heart rate actually matters more right now.

The Algorithm Matters as Much as the Sensor

But here's the thing: the heart rate monitor only receives a raw signal of light intensity. And then something else must translate this information into a proper value. This translation is made using proprietary software for each brand separately. This is what results in the daily scores and graphs that we're all so obsessed with.

The task of this software is to determine whether the signal represents the heartbeat or some noise created by physical activity, skin contact, or ambient light.

A 2024 study published in Sensors concluded that the heart rate readings for the Oura Ring remained accurate compared to ECG when lower quality readings were included.

The case of heart rate variability was quite different. Heart rate variability measurements became less accurate when the device used lower-quality data, especially for people over the age of 45.

The heart rate algorithm used in WHOOP underwent a huge update in February 2026. The company used the training data collected at its own research facility for a wide range of skin tones, body types, and activity levels. Also, it used cloud-based processing for data analysis after the workout or a sleep session instead of edge processing.

This is a completely different strategy from having the watch do all the calculations on the device itself. Therefore, two devices that have identical hardware will give different figures from the same data depending on the interpretation of the software.

That’s also the reason why your figures might be altered by the brand itself even when it hasn't altered any of its hardware. With the release of the February 2026 update by WHOOP, there were some changes seen in heart rates and recovery scores for all 4.0 and 5.0 users, although none of the users’ activity had changed at all. The sensors remained the same, but the math behind them wasn’t.

The same happened to me with my step count on my Ultrahuman Ring AIR after a firmware update. If you've noticed your own numbers drift after an app update with no real change in your routine, this is usually why.

Skin Tone and Wrist Size Add Another Layer

There are more than just sensors, positioning, and algorithms involved, of course. There's also the human who wears the device, plus PPG sensors do not work the same for all people.

The amount of light absorbed by the skin is dependent upon the amount of melanin in the skin, meaning you'll get different results on different skin tones.

One of the most extensive analyses is offered by Koerber et al. (2023). They showed that this may lower the accuracy of reading in darker skin tones, not just for a particular brand, but for many brands.

PPG works by shining light into the skin and measuring the light reflected back. Since melanin absorbs more light, darker skin tones can reduce the amount of reflected light that reaches the sensor. This can lower the signal quality and, in some situations, reduce measurement accuracy.

The same goes if you have any tattoos on the area where the wearable sensor is placed. Dark or densely pigmented tattoo ink may absorb or scatter the emitted light, making it more difficult for the sensor to obtain a clean signal.

This doesn't necessarily prevent measurements altogether, but it can increase the likelihood of inaccurate or inconsistent readings.

In addition to this, the circumference of your wrist along with the device's tightness and fit will also have an impact on the amount of light that leaks out of the sensor rather than being reflected back into it. A watch that's too loose allows ambient light to enter and reduces the amount of reflected light captured by the sensor, while one that's too tight can alter blood flow beneath the sensor. Smaller wrists may also make it harder for the sensor to maintain consistent contact with the skin.

For these reasons, manufacturers generally recommend wearing the device snugly, but not tightly and positioning it slightly above the wrist bone during measurements.

Activity Type Throws Everything Off Differently for Each Brand

Resting heart rate during sleep is one thing, but your heart rate while working out is a totally different ball game for the sensors. This is where the difference becomes most evident between gadgets.

Motion artifacts are caused by movement, which means that your swinging arm interferes with the ability of the device to measure the reading the sensor is trying to capture. All companies have their algorithms to filter these movements and some handle certain movement patterns better than the others.

That's also the reason why chest strap monitors (that treat electrical signals directly from your heart rather than relying on light through skin) still outperform every wrist or finger-based wearable during high-intensity training.

What This Means If You're Comparing Numbers With a Friend

When you and your friend are wearing different brands of wearables and your numbers don’t line up, that’s perfectly normal and not an indicator that your device is faulty. All that really matters is consistency over time.

The resting heart rate trend on your Oura or Garmin should be giving you an accurate picture of your recovery and stress based on your personal baseline. Consistently improving your workouts and striving towards a better trend in HRV is better than a single measurement.

My honest takeaway here is that heart rate tracking on consumer wearables has definitely improved, especially in terms of resting and steady state numbers. But there are still significant differences between brands and these are very much real. They come from design choices, not manufacturing defects.

The final number relies on three factors: the placement of the sensor, how often readings are taken, and how the softtware processes that information.

But will I stop arguing with my partner on how he stresses me out, which is why my heart rate drops later than his at night? Probably not.