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That makes it useful for tracking your personal trend over time—not for replacing a laboratory test or diagnosing a heart or lung condition. Here’s how to get a reading, find it in Apple Health, understand the number, and troubleshoot missing or unusual results.
What is VO₂ max?
VO₂ max is the maximum amount of oxygen your body can use during intense exercise. It is commonly used as a measure of aerobic, or cardiorespiratory, fitness and is expressed in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min). A higher value is generally associated with better aerobic capacity, although age, sex, body size, genetics, training history, illness, medication, and testing conditions all affect the result.
A clinical or sports-laboratory test measures oxygen consumption directly. You exercise progressively harder on a treadmill or bicycle while breathing through equipment connected to a metabolic analyser. Apple Watch works differently: it collects wearable-sensor data during submaximal activity and calculates an estimate. See Apple’s explanation of the HealthKit VO₂ max quantity.
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- A FULL-FEATURED HEALTH TRACKER—Apple Watch Series 11 gives you invaluable insights about your body, right from your wrist. Take an ECG anytime* and get alerts for a high and low heart rate or an irregular rhythm*—so you can stay closer to your heart. You can also view your overnight health tracking metrics with the Vitals app* and be notified of possible sleep apnea.*
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What “Cardio Fitness” means on Apple Watch
Apple’s consumer-facing name is Cardio Fitness; the underlying HealthKit metric is estimated vo2Max. During supported outdoor workouts, the watch combines heart-rate response with movement, GPS, and information such as age, sex, height, and weight. The goal is to infer how efficiently your body is working during exercise without requiring an all-out laboratory test.
Heart-rate quality matters. If the watch cannot obtain a dependable pulse signal, or if the route and activity do not match the conditions used by the algorithm, it may not create a reading.
Which Apple Watch models support VO₂ max?
Apple says Apple Watch Series 3 or later can record an estimated VO₂ max. The feature is part of the Apple Watch and Apple Health experience; it does not require an Apple Fitness+ subscription. Software availability and some interface details can vary by iOS, watchOS, device, and region, so use Apple’s current Cardio Fitness support page for device-specific requirements.
A more expensive watch is not automatically a more accurate VO₂ max device. If you already own a compatible Apple Watch, upgrading solely to obtain Cardio Fitness is difficult to justify. Battery life, GPS performance, durability, cellular connectivity, and other training features are more meaningful reasons to choose a newer or premium model.
Which workouts count?
Apple documents these qualifying activities:
- Outdoor Walk
- Outdoor Run
- Hiking
For the best chance of receiving an estimate, use an open outdoor route with good GPS reception, reliable heart-rate tracking, and relatively flat terrain. Apple’s technical documentation describes a terrain requirement of less than 5% incline or decline and an exertion condition of approximately 130% or more of resting heart rate. These are system criteria, not a promise that you can force a result by hitting an exact number.
Indoor workouts, treadmill sessions, cycling, swimming, strength training, and push workouts should not be expected to produce this Cardio Fitness estimate. Cyclists in particular should not assume that an outdoor cycling workout will generate the same VO₂ max data as an outdoor run or walk. Steep hiking routes and heavily obstructed areas can also prevent a reading.
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How to get your first Cardio Fitness reading
- Wear the watch for at least a day. Apple says the system may need at least 24 hours of wear, baseline information, passive measurements, and several qualifying workouts.
- Complete your Health Details. On the paired iPhone, open Health, tap your profile picture, then check details such as date of birth, sex, height, and weight. Correct information improves the relevance of the estimate.
- Wear the watch snugly. It should maintain consistent skin contact without being uncomfortably tight.
- Go outdoors. Choose an open, relatively flat route where GPS reception is good.
- Open Workout on Apple Watch. Select Outdoor Walk, Outdoor Run, or Hiking.
- Exercise at a meaningful, sustained pace. An extremely easy, short, interrupted, or sensor-poor session may not qualify.
- Repeat the activity. The first workout does not itself generate a VO₂ max sample according to Apple’s developer documentation, so do not assume a missing value means the feature is broken.
Where to find VO₂ max on iPhone
Open the Health app and follow:
Search → Heart → Cardio Fitness
From there, you can view day, week, month, six-month, and year trends. Labels may differ slightly by software version or region. If you have more than one source writing data to Health, review the Cardio Fitness data sources and access settings so you know which device or app supplied each entry.
How to interpret your Apple Watch number
Use the trend, not one reading
The most useful question is usually not “Is this one number perfect?” but “What direction is my number moving under similar conditions?” A sustained change over several weeks is more informative than a single estimate. Compare it with practical indicators such as your pace, walking or running comfort, recovery, and ability to sustain exercise.
A small rise or fall can reflect ordinary measurement noise. Heat, dehydration, poor sleep, illness, unusual fatigue, and a change in medication can all affect heart-rate response or exercise performance. A lower estimate during a temporary illness does not necessarily mean a permanent loss of fitness.
Understand Apple’s classifications
For users aged 20 and older, Apple Health can compare Cardio Fitness with age- and sex-based classifications including:
- Low
- Below Average
- Above Average
- High
These are reference classifications, not universal athletic rankings or medical thresholds. A “High” label does not guarantee health, and a “Low” label does not diagnose disease. Avoid applying generic VO₂ max tables from another website as though they were identical to Apple’s categories.
Remember that the value is relative to body weight
Because the unit is mL/kg/min, body mass is part of the calculation. A substantial weight change can move the relative value even if your absolute oxygen-use capacity has changed less—or not at all. This is one reason to interpret the number alongside real-world performance and longer-term context.
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- A FULL-FEATURED HEALTH TRACKER—Apple Watch Series 11 gives you invaluable insights about your body, right from your wrist. Take an ECG anytime* and get alerts for a high and low heart rate or an irregular rhythm*—so you can stay closer to your heart. You can also view your overnight health tracking metrics with the Vitals app* and be notified of possible sleep apnea.*
- KNOW YOUR SLEEP SCORE—With advanced sleep tracking, Series 11 gives you a daily sleep score. It’s an easy way to help measure and understand the quality of your sleep, so you can make it more restorative.
- GET HYPERTENSION NOTIFICATIONS—Did you know that Series 11 can spot signs of chronic high blood pressure? By analyzing how your blood vessels respond to beats of the heart, it can notify you of possible hypertension.*
- SAFETY FEATURES—A smartwatch that puts your safety first. Series 11 can detect a hard fall or severe car crash, automatically help connect you with emergency services, and notify your emergency contacts.* Check In can automatically notify a loved one when you’ve arrived at your destination.
- A POWERFUL FITNESS TRACKER—With advanced metrics and motivating features like Pacer, Heart Rate Zones, training load, and Workout Buddy powered by Apple Intelligence from your nearby iPhone*—Series 11 is made to help everyone stay active, from runners and hikers to strength-training enthusiasts.
Why is VO₂ max useful?
It provides a convenient baseline
Cardio Fitness gives you a repeatable starting point for observing your own aerobic fitness without booking a test. Establishing a baseline can make changes in activity, recovery, or health easier to notice.
It can show training direction
A rising multi-week trend, especially when accompanied by improving walking or running performance, is consistent with improving aerobic capacity. Apple says many people can improve VO₂ max by increasing the intensity and frequency of cardiovascular exercise. Build activity gradually and allow appropriate recovery rather than chasing a single watch value.
It can highlight a sustained decline
A persistent fall may reflect reduced activity, illness, recovery problems, inaccurate Health Details, or a workout and sensor issue. It is a prompt to investigate—not proof of a particular cause.
It can encourage regular exercise
A simple trend can make cardiovascular activity more tangible. Used sensibly, it can support consistency without turning every workout into a test.
Why Apple Watch may not show a VO₂ max estimate
Check these common causes:
- The workout was indoors or performed on a treadmill.
- You selected cycling, swimming, strength training, or another unsupported workout type.
- You used a push workout, which Apple explicitly excludes.
- The activity was too short, too easy, heavily interrupted, or did not produce enough exertion.
- The route was steep, obstructed, or affected by weak GPS.
- The watch was loose or heart-rate data was missing or implausible.
- Height, weight, date of birth, sex, or other Health Details are incomplete or outdated.
- You have not worn the watch long enough to establish baseline information.
- You have completed only the first qualifying workout.
- A software or algorithm change has affected how historical estimates are displayed or recalculated.
- A health condition or medication has changed your heart-rate response.
Recovery checklist
- Update your Health Details, including current weight and height.
- Restart both the Apple Watch and paired iPhone.
- Check that Workout and Health permissions are enabled.
- Calibrate the watch using Apple’s calibration guidance.
- Complete several outdoor walks or runs on a relatively flat route with good GPS.
- If readings exist but appear unexpected, inspect Cardio Fitness data sources and access settings in Health.
- Do not delete historical data simply because one estimate looks unusual.
If the watch continues to report unexpectedly low fitness and that conflicts with your symptoms or known ability, discuss it with a healthcare professional.
Medications, health conditions, and sensor limitations
Apple warns that conditions or medications that limit heart rate can cause an overestimation. Its technical documentation also describes identifying heart-rate-limiting medication in Health to improve the estimate. Keep medication information current, but never stop, change, or reinterpret prescribed medication because of a watch reading.
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- GET SMARTER HEALTH INSIGHTS—Apple Watch SE 3 is an advanced health tracker that helps you better understand your body. The smartwatch sends alerts for an irregular heart rhythm, or an unusually high or low heart rate.* You can view your overnight health tracking metrics with the Vitals app,* track your sleep score, and get sleep apnea notifications.*
- A GREAT FITNESS TRACKER—SE 3 can track your workouts in a healthy number of ways—from running and strength training to simply achieving your 10K steps a day. With real-time metrics and Workout Buddy powered by Apple Intelligence from your nearby iPhone,* you’ll hit your goals like never before.
- KEEP HELP CLOSE AT HAND—SE 3 is a smartwatch that can detect a hard fall or severe car crash and automatically help connect you to emergency services and notify your emergency contacts.* Check In can automatically notify a loved one when you’ve arrived at your destination.
- WOMEN’S HEALTH INSIGHTS—SE 3 has advanced sensors that track your wrist temperature while you sleep. With this smartwatch, you can log your periods and symptoms, get period predictions, and get notified of cycle deviations.*
- GREAT BATTERY LIFE AND FASTER CHARGING—Enjoy a smartwatch with all-day, 18-hour battery life* from sunup to sundown. Then charge up to twice as fast as SE 2* and get up to 8 hours of battery in just 15 minutes.*
Irregular heart rhythms, cardiovascular disease, and other conditions can make a heart-rate-based estimate harder to interpret. Loose fit, poor contact, cold skin, motion artefacts, sweat, tattoos, or poor circulation may also degrade the pulse signal. These are reasons to treat an unusual result cautiously, not reasons to diagnose yourself.
Symptoms matter more than the displayed number. Seek medical attention for symptoms such as chest pain, fainting, unusual or severe breathlessness, palpitations, or severe fatigue, particularly during exercise.
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The evidence supports a careful middle position: the estimate can be useful, but individual accuracy is not guaranteed.
In Apple’s validation material, the company reports an average error of less than 1 MET and test–retest reliability above 0.85 in its validation data, with results comparable to commonly used submaximal exercise testing. Read the Apple validation document for its population and protocol details.
Independent research has found wider individual disagreement. A 2025 PLOS One study of 30 participants reported that Apple Watch underestimated laboratory VO₂ max by an average of 6.07 mL/kg/min. It found a mean absolute error of 6.92 mL/kg/min, a mean absolute percentage error of 13.31%, and limits of agreement from −6.11 to 18.26 mL/kg/min. Another small Apple Watch Series 7 study reported a laboratory mean of 45.88 mL/kg/min compared with a predicted mean of 41.37 mL/kg/min.
Those findings are not automatically interchangeable: studies differ in watch generation, software, population, activity protocol, and laboratory method. The practical conclusion is straightforward:
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- A FULL-FEATURED HEALTH TRACKER—Apple Watch Series 11 gives you invaluable insights about your body, right from your wrist. Take an ECG anytime* and get alerts for a high and low heart rate or an irregular rhythm*—so you can stay closer to your heart. You can also view your overnight health tracking metrics with the Vitals app* and be notified of possible sleep apnea.*
- KNOW YOUR SLEEP SCORE—With advanced sleep tracking, Series 11 gives you a daily sleep score. It’s an easy way to help measure and understand the quality of your sleep, so you can make it more restorative.
- GET HYPERTENSION NOTIFICATIONS—Did you know that Series 11 can spot signs of chronic high blood pressure? By analyzing how your blood vessels respond to beats of the heart, it can notify you of possible hypertension.*
- SAFETY FEATURES—A smartwatch that puts your safety first. Series 11 can detect a hard fall or severe car crash, automatically help connect you with emergency services, and notify your emergency contacts.* Check In can automatically notify a loved one when you’ve arrived at your destination.
- A POWERFUL FITNESS TRACKER—With advanced metrics and motivating features like Pacer, Heart Rate Zones, training load, and Workout Buddy powered by Apple Intelligence from your nearby iPhone*—Series 11 is made to help everyone stay active, from runners and hikers to strength-training enthusiasts.
- Use Apple Watch for convenient relative tracking.
- Do not treat one estimate as your definitive physiological VO₂ max.
- Do not call the feature clinically accurate without specifying the population, protocol, and error.
- Use a laboratory test when precision is important.
How to improve your VO₂ max
Regular cardiovascular exercise, gradually increased in frequency and appropriately challenging intensity, can improve aerobic fitness for many people. Consistency, recovery, sleep, and sensible progression matter more than forcing a high reading after one hard session.
If you are inactive, have a medical condition, take heart-rate-limiting medication, or have concerning symptoms, ask a healthcare professional how to begin safely. The Apple Watch cannot establish whether intense exercise is safe for you.
When a laboratory test or medical advice makes sense
Choose a properly administered laboratory or sports-performance test when you need a direct measurement—for example, detailed athletic assessment, research, or a clinical evaluation. It is more expensive, less convenient, and physically demanding, but it measures respiratory gases rather than estimating them from wearable data.
Seek medical advice for a persistent unexplained decline, a low result that concerns you, or any exercise-related symptoms. A watch estimate should add context to a clinical conversation, not replace one.
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Which Apple Watch should you buy for Cardio Fitness?
If Cardio Fitness is your only reason for buying a watch, do not assume you need the most expensive model. Apple’s stated support begins with Series 3 or later.
- Apple Watch SE 3: a budget-oriented choice for an iPhone user who wants the Apple Watch ecosystem and basic fitness tracking.
- Apple Watch Series 11: the most practical all-round choice for someone who wants Cardio Fitness alongside everyday smartwatch features.
- Apple Watch Ultra 3: best considered for battery life, rugged construction, navigation, and endurance activities—not for a guaranteed improvement in VO₂ max accuracy.
Choose based on compatibility, battery requirements, GPS and outdoor use, durability, cellular needs, other health features, and budget. Apple Watch requires an iPhone, while Garmin, Polar, COROS, and Suunto watches may better suit people who prioritise long battery life or specialised running and cycling tools. Their VO₂ max figures use different algorithms and should not be compared directly without comparable evidence.
Apple Fitness+ is optional workout content; it is not required for Cardio Fitness.
Bottom line
Apple Watch Cardio Fitness is best used as a convenient, consistent personal trend. Wear the watch correctly, use supported outdoor workouts, keep Health Details current, and judge changes across multiple weeks. For a definitive number—or for medical decisions—use a qualified clinician or laboratory test, not a single smartwatch estimate.
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