Apple Watch does not measure your VO2 max; it estimates it. The estimate, which Apple calls Cardio Fitness, comes from how your heart rate responds to pace on outdoor walks, runs and hikes. In independent lab comparisons it was off by about 13 to 16 percent on average, usually reading low, and it is much better at showing whether you are getting fitter than at giving your exact number. Steady aerobic training over weeks to months is what raises it.
- 13–16%average gap between Apple Watch and a lab test to exhaustion, usually reading lowCaserman 2024; Lambe 2025
- +4.9–5.5ml/kg/min from endurance or interval training across 28 controlled trialsMilanović et al. 2015
- 1.5 ml/kg/minchange before Aera calls your cardio fitness Fitter or EasingAera
For what VO2 max is in the first place, see VO2max on a watch. This guide covers the Apple Watch number itself.
How Apple Watch estimates Cardio Fitness
A lab measures VO2 max with a mask while you run or cycle to exhaustion, in millilitres of oxygen per kilogram per minute (ml/kg/min). According to Apple's white paper, the watch instead compares your heart rate against your speed during an Outdoor Walk, Outdoor Run or Hike, and projects from that submaximal effort where your ceiling would be, using your age, sex, weight and height from the Health app.
The conditions are strict. The ground has to be fairly flat (less than about a 5% grade), the GPS and heart-rate signals have to be good, and you have to work hard enough to lift your heart rate roughly a third of the way from resting to maximum. Indoor workouts do not count, the first qualifying workout produces no value, and Apple Support says estimates run from 14 to 65 ml/kg/min.
From age 20, the Health app shows which level your estimate falls in for your age and sex. You can opt in to a Low Cardio Fitness notification. Apple sets that threshold at the lowest fifth of the FRIEND lab registry for ages 20 to 59, and at 18 ml/kg/min for men and 15 for women from age 60, and only notifies when the value is consistently low.
How accurate is it?
Apple validated the algorithm on 221 people whose data was held back from design. The average error was 1.4 ml/kg/min, with a spread of about 4.7 either way. But Apple's reference was itself projected from submaximal lab tests, not a full test to exhaustion.
The watch reads low, but it rises with you.
Independent studies that ran people to exhaustion found larger gaps. Caserman and colleagues (2024) tested 19 adults with an Apple Watch Series 7 against a lab cycling test: the watch read lower on average (41.4 against 45.9 ml/kg/min), was off by about 16% on average, and tended to over-read less fit people and under-read very fit ones. Lambe and colleagues (2025) tested 28 adults, mostly on Series 9 and Ultra 2, against a treadmill test: the watch read 6.1 ml/kg/min low on average, about 13% off, and for some people it was off by well over 10 points.
So the number can land a level away from a lab result. Your own trend, fed regular outdoor sessions, is the part to trust.
What is a good VO2 max for your age?
Apple's levels come from FRIEND, a US registry of treadmill tests (Kaminsky and colleagues, 2015). The middle value (50th percentile) in that registry, in ml/kg/min:
- 20–29: men 48.0, women 37.6
- 30–39: men 42.4, women 30.2
- 40–49: men 37.8, women 26.7
- 50–59: men 32.6, women 23.4
- 60–69: men 28.2, women 20.0
- 70–79: men 24.4, women 18.3
Above the middle means fitter than half the people your age and sex who took the test, and the registry is a group of people who chose to be tested, not the whole population. Because the watch tends to read under a treadmill test, a value just under the middle may really be at it.
Why it reads low, or barely moves
Apple's paper lists what makes the estimate less accurate:
- Heat, caffeine, dehydration and altitude. Anything that lifts your heart rate at a given pace makes you look less fit. More in why heat raises your heart rate.
- Hills, sand and loads. Steep ground does not qualify, and a heavy backpack, a child, a stroller or soft ground add work the watch cannot see.
- Weak GPS or a loose watch. Speed and heart rate both have to be clean for a workout to count.
- Wrong details in Health. An out-of-date age, weight or height skews every estimate.
- Heart-rate-lowering medicines. Beta blockers and calcium channel blockers push the estimate up if not entered. In Apple's validation, entering them in Health cut the average error from 11.8 to 1.6 ml/kg/min.
It also moves slowly by design: training changes VO2 max over weeks, and the watch only updates from qualifying workouts. If you mostly cycle, swim or lift, the number can sit still. That is a gap in what the watch sees, not proof your fitness stopped.
How to raise it
Milanović and colleagues (2015) pooled 28 controlled trials of healthy adults aged 18 to 45. Compared with no exercise, continuous endurance training raised VO2 max by about 4.9 ml/kg/min and high-intensity interval training by about 5.5, with intervals a small step ahead. Longer programmes did better.
VO2 max is built over weeks and months, so a steady habit beats any one big week.
- Build regular aerobic volume. Most weeks, several sessions of steady walking, running, cycling or similar at an effort you could keep up while talking in short sentences. See what zone 2 training is.
- Add some intervals. Once you have a base, one or two sessions a week of hard efforts with easy recoveries. See what intervals are.
- Give the watch something to read. A brisk, flat outdoor walk or run each week keeps the estimate fresh.
- Give it time. Expect weeks to months. VO2 max also falls quickly with inactivity, so consistency beats any one big week.
Why it matters for health
Cardio fitness is one of the stronger long-term health signals. Kodama and colleagues (2009) pooled 33 studies of over 100,000 healthy adults: each extra MET of fitness (3.5 ml/kg/min) went with about 13% lower all-cause mortality and 15% fewer heart and circulation events. Mandsager and colleagues (2018) followed 122,007 people after treadmill tests and found that higher fitness kept going with lower mortality, with no point where the benefit stopped. These are associations in large groups, not a forecast for one person, and much of the difference sits between the least fit group and the next one up.
When to see a doctor
- Repeated Low Cardio Fitness notifications, especially if everyday activities feel harder than they used to.
- Breathlessness, chest pain, dizziness or fainting during exercise, whatever the number says.
- Before hard training if you have a heart condition or take heart medicines. This page is general information, not medical advice.
How Aera does this
VO₂ max on a runOn an outdoor run or walk, Aera prints the estimate Apple Health recorded that day and how far it has moved against the reading about a month earlier.
Aera, for iPhone and Apple Watch
Aera reads the Cardio Fitness estimates your Apple Watch saves to Apple Health and holds them against you, not against a single day. On Home, a VO2 max tile appears when your latest estimate moves away from your usual of the last six weeks, or when you keep opening it. It says Typical, Below your usual or Notably low (or the same above), and until there are five estimates it says Building baseline instead of guessing. When there are few estimates, its sheet says the trend fills in from outdoor workouts.
On the Fitness tab, Progress shows Your fitness: your cardio fitness against where you stood twelve weeks ago. It waits for six estimates spread over at least four weeks, and only calls a change of 1.5 ml/kg/min or more Fitter or Easing; anything smaller reads Holding. Under it, a level from Low to High places your estimate among people of your age and sex in the FRIEND registry, adjusted for the watch reading about 10% under a treadmill test. The Field Guide explains each step, including why the level can be a band off and why hills, heat and medicines blur the estimate. There is no account, and health numbers are worked out on your iPhone. These reads are free.
Aera is not a medical device.
Quick answers
Is Apple Watch VO2 max accurate?
It is a fair estimate, not a measurement. Apple's own validation found an average error of about 1.4 ml/kg/min, but independent lab studies found the watch reading about 4 to 6 ml/kg/min below a real test on average, with individual errors larger still. It tracks your own trend better than it matches a lab number.
What is a good VO2 max for my age?
It depends on age and sex. In the FRIEND registry of treadmill tests, the middle value is about 48 ml/kg/min for men and 38 for women in their twenties, falling to about 33 and 23 in their fifties. Apple Watch tends to read somewhat lower than a treadmill test, so treat the Health app's level as a rough guide and your own trend as the real signal.
Why is my Apple Watch cardio fitness low?
It may be low, or it may be under-read. Heat, hills, a weak GPS signal, carrying a load, caffeine, dehydration and wrong age, weight or height in the Health app can all pull the estimate down. Heart-rate-lowering medicines push it the other way unless you enter them in Health. Several flat outdoor walks or runs at a steady effort give the cleanest estimate.
How can I improve my VO2 max?
Regular aerobic exercise, kept up for weeks to months. A meta-analysis of controlled trials found continuous endurance training raised VO2 max by about 4.9 ml/kg/min and interval training by about 5.5. Most weeks can be steady, easy-to-moderate cardio, with one or two harder interval sessions once you have a base.
Related field notes: VO2max on a watch · what zone 2 training is · what intervals are · why heat raises your heart rate · what a consumer watch cannot measure
Related guide: Apple Watch HRV: what is normal, and why is mine low?
Sources
- Apple Inc. Using Apple Watch to Estimate Cardio Fitness with VO2 max. White paper, May 2021. apple.com/healthcare.
- Apple Inc. Track your cardio fitness levels. Apple Support. support.apple.com/en-us/108790, accessed October 2026.
- Caserman P, Yum S, Göbel S, Reif A, Matura S. Assessing the accuracy of smartwatch-based estimation of maximum oxygen uptake using the Apple Watch Series 7: validation study. JMIR Biomedical Engineering. 2024;9:e59459.
- Lambe R, O'Grady B, Baldwin M, Doherty C. Investigating the accuracy of Apple Watch VO2 max measurements: a validation study. PLOS ONE. 2025;20(5):e0323741.
- Kaminsky LA, Arena R, Myers J. Reference standards for cardiorespiratory fitness measured with cardiopulmonary exercise testing: data from the Fitness Registry and the Importance of Exercise National Database. Mayo Clinic Proceedings. 2015;90(11):1515-1523.
- Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials. Sports Medicine. 2015;45(10):1469-1481.
- Kodama S, Saito K, Tanaka S, et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis. JAMA. 2009;301(19):2024-2035.
- Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open. 2018;1(6):e183605.