Especially for athletes, VO2 Max is a metric of key importance for cardiovascular fitness. Many smartwatches, such as the Apple Watch, give estimates of VO2 max based on heart rate and pace. But if you’re someone like me who is generally working towards a healthy lifestyle, this measurement can be quite confusing and, at times, discouraging.
So, when I started focusing on my Apple Watch, giving me a lower VO2 max, not only did I get concerned, but I set out on my personal research to find if this number was accurate and if I needed some serious TLC, or if this was just a fluke.
Quick Answer: Is Apple Watch VO2 Max Accurate?
Apple Watch VO2 max, which appears as ‘Cardio Fitness’ on the Apple Health app, is actually an estimation and not a lab test for VO2 max. While Apple’s estimation may come in handy for observing cardio fitness trends over the years, scientific studies have found that it can both underestimate and overestimate VO2 max values relative to the lab tests.
According to Apple, Apple Watch Series 3 or newer models can calculate VO2 max through heart rate and motion measurements during outdoor walking, outdoor running, and hiking. This service caters to users above 20 years of age whose cardio fitness varies from 14 to 65 mL/kg/min.
Apple Watch VO2 Max vs Lab VO2 Max
| Measurement Type | What It Means | Accuracy Level | Best Use |
| Lab VO2 max / CPET | Measures oxygen use during exercise with gas analysis | Highest | Clinical and athletic testing |
| Apple Watch VO2 max / Cardio Fitness | Estimates VO2 max using heart rate, GPS, motion, and personal data | Moderate estimate | Tracking long-term fitness trends |
| Field test estimate | Uses formulas from walking/running performance | Variable | General fitness benchmark |
| Personal feeling of fitness | Subjective impression | Low | Motivation and context only |
What the studies say:
- A study by JMIR Publications found that the Apple Watch underestimated VO₂ max with a high mean absolute percentage error of 15.79%, indicating that there is a noticeable difference from lab measurements.
- Another study by West Texas A&M University concluded that the Apple Watch’s VO₂ max estimates were not very different from VO₂MasterPro measurements; however, they had a very weak correlation (r=0.2), indicating poor agreement.
- The same study also concluded that the Apple Watch overestimated VO₂ max by an average of 6 mL/kg/min in six cases but underestimated it by 10.4 mL/kg/min in ten cases, and when analyzed by sex, Apple’s estimates were fairly accurate for males but not for females.
- but not for females.
What this means for you
The studies indicate that the Apple Watch VO2 max measurement is directionally useful but not medically accurate. For some people, it might be reasonably accurate, whereas for others, the difference between the two measures is quite significant and can make the score unreliable.
How the Apple Watch measures VO2 Max
Simply put, VO2 max is the maximum oxygen consumed by us during intense exercise. This is a great indicator of the cardiovascular health of an individual. Some health experts even swear by this indicator.
While normally, it is calculated in milliliters of oxygen a minute per kilogram of body weight, the Apple Watch uses heart rate and motion sensors during a workout like an Outdoor Walk, Outdoor Run, or Hiking on the Workout app on Apple Watch. This metric can be calculated using any Apple Watch Series 3 or later.
The Apple Watch also takes into consideration my personal data, such as age, weight, gender, height, and any medications I take that can affect my heart rate.
Apple calls this Cardio Fitness
Apple puts this feature under “Cardio Fitness” in the Health app. In contrast to the measurement conducted in a lab, measuring the oxygen intake through a mask, Apple Watch calculates this indicator on the basis of the user’s heart rate and body movements.
Supported Apple Watch models
According to Apple, VO2 max estimation is possible for all Apple Watch Series 3 and newer models.
Supported workout types
According to Apple, VO2 max estimation is performed during:
- Outdoor Walk
- Outdoor Run
- Hiking
Also, the company specifies that the most accurate results are obtained in cases where GPS signals are sufficiently strong and there is a stable heart-rate monitoring on flat open air surfaces. At the same time, my personal information, including age, weight, gender, height and any medications that may affect heart rate, are also taken into consideration.
Which Workouts Count for Apple Watch VO2 Max?
| Workout | Usually Updates VO2 Max? | Why |
| Outdoor Walk | Yes | Supported by Apple |
| Outdoor Run | Yes | One of the main supported workouts |
| Hiking | Yes | Supported with GPS and HR data |
| Indoor Run / treadmill | Usually no | Lacks outdoor GPS conditions |
| Cycling | Usually no | Not listed by Apple for VO2 max estimation |
| HIIT | Usually no | Irregular pace and movement |
| Elliptical | Usually no | Indoor machine movement |
| Strength training | No | Not a cardio fitness estimation workout |
| Rowing | No | Not officially supported |
My experience with the Apple Watch’s VO2 Max:
Judging by medical-grade accuracy, the Apple Watch is not a replacement for a proper VO2 max test. This not only gave me some peace of mind and relief, but it also made it more concrete that fitness devices are not to be blindly followed. But I wasn’t done here; I tested out my VO2 max while doing workouts with my Apple Watch for a period of time to see if it improves, or should I just stop bothering about this number altogether.
At first, the VO2 max was seriously low, and I couldn’t fathom why. I’m not a fan of cardio, but I do get through a HIIT workout without feeling like I wanna die. So there was my first indication that the number being displayed on my Health app was not correct.
But I wasn’t letting intuition lead the way, so I thought of training more and including more cardio in my rotation. Now, that would help me get this number up, right? Wrong! Not only was my Apple Watch underestimating my VO2 max, but it was going further down even after all that training.
According to my Apple Watch, my VO2 max was 28.6, which is considered ‘low.’ After several days of incorporating cardio into my gym routine, my VO2 max came down to 27.1, while in theory, it should’ve increased.
Although I have seen people test the Apple Watch’s VO2 max with a treadmill test, the results were pretty close. However, it is important to understand that the VO2 max data is only calculated by the Apple Watch with 20+ minutes of uninterrupted outdoor workouts.
So if my Apple Watch misses a reading because of its fit or I stop in the middle of my workout to play with a dog or something, that is going to affect my VO2 max reading. Not to say I’m way healthier than what my Apple Watch suggests, but I’m sure it is giving me a ballpark figure of my VO2 max, not the actual reading.
Why Is My Apple Watch VO2 Max So Low?
There are multiple factors that could result in your Apple Watch having a very low VO2 max reading despite your fitness level.
Most of your workouts are unsupported
If you are only exercising indoors, cycling, lifting weights, or using HIIT programs, your Apple Watch might not get enough pace data from outdoor activities to calculate Cardio Fitness.
Incorrect information in Health Details
This includes your age, gender, height, and weight which will be considered by Apple in the calculations of VO2 max.
Medication settings might have an effect
Apple includes medications that may affect heart rate when calculating your cardio fitness score. If this feature is mistakenly activated, it will give you a wrong estimate.
Weak signal from heart rate tracking
This could be due to a poor fit of the band, tattoos, cold hands, sweating and moving around during exercise.
Terrain and GPS could alter VO2 max results
This includes hills, traffic lights, bad GPS reception and uneven paces during exercises.
Why Is My Apple Watch VO2 Max Dropping Even Though I’m Fitter?
This is actually a pretty typical problem reported by Apple Watch users. Causes may be:
- You switched between age groups
- Your weight has changed in your Apple Health app
- You developed better cycling and strength performance as opposed to your running fitness
- Outdoor exercises become less continuous and involve heat/hills/slow pace
- Your watch fit has changed
- You engage in fewer qualifying outdoor exercise sessions
My point is that my HIIT and gym workouts were contributing to my improved fitness, which my Apple Watch could not adequately assess with its VO2 max formula.
How to improve the accuracy of VO2 max readings on the Apple Watch
Even if it’s a ballpark figure, to get the most out of your Apple Watch’s VO2 max readings:
- Make sure the watch is snug on your wrist and not loose.
- Regularly update your personal data on your Health app.
- Make sure to pause your workouts on the Apple Watch whenever you take a break.
- Make sure to stop the workouts on the Apple Watch as soon as your main workout is over, right before cool down and stretching.
- Recalibrate your Apple Watch regularly for the most accurate readings.
- Make sure to check that you haven’t added any medications that can affect your heart rate if that is not the case.
How to check medication settings
To check this, head to Watch > Health > Health Details > Medications That Affect Heart Rate > if it is 0, it means there are no medications added.
How to Calibrate Apple Watch for Better VO2 Max Estimates
- Open Settings on your iPhone.
- Tap on Privacy & Security > Location Services > System Services.
- Turn on Motion Calibration & Distance.
- Go to an outdoor area with a clear GPS signal.
- Start an Outdoor Walk or Outdoor Run workout.
- Walk or run at your normal pace for around 20 minutes.
Calibration of your Apple Watch can improve pace, distance and fitness estimation accuracy.
Should You Trust Apple Watch VO2 Max?
| Use Case | Trust Apple Watch? |
| Tracking long-term cardio trends | Yes, cautiously |
| Medical diagnosis | No |
| Athletic testing baseline | Lab test is better |
| Outdoor running improvement | Useful |
| Cycling or HIIT fitness | Limited |
| Sudden one-week drop | Be cautious |
When Should You Get a Lab VO2 Max Test?
A VO2 max test in the lab or CPET is still the gold standard. Professional testing is recommended for those who:
- Are an elite athlete
- Have a low Apple Watch VO2 max reading
- Have a sudden drop in VO2 max for no reason
- Got a Cardio Fitness warning from their watch
- Need an accurate fitness level determination
- Have any cardiovascular problems
Wrap Up
As with any other metrics that are only accurately measured using lab tests as a gold standard, Apple Watch gives a ballpark estimate and shouldn’t be taken too literally. However, these are good for the general upkeep of your health.
Personally, I have strived to get my Apple Watch’s VO2 max graph to go higher, but maybe it’s the breaks I take between my workouts that are not giving me the desired results. Independent studies have shown that the precision of VO2 max calculated by Apple Watch may differ significantly from that observed in the lab. Although this alone is not enough for considering the tool completely pointless, this surely does not mean that one needs to panic if there is a drop in VO2 max performance.
As a way to track cardio fitness, the Apple Watch may be useful, but when it comes to more serious purposes such as VO2 max measuring or medical diagnostics, you would still need to take a professional test and consult your doctor.
FAQs
Author
I appreciate the work you put into this. I had just looked at the vo2max numbers my Apple watch had generated from several runs a few years ago. I too was shocked by how low the numbers were as I can walk longer and run farther than the very large majority of my peers. I had given up wearing the watch after those runs because many things were problematic. The watch functions were a pain, flawed and really a nuisance to use. Also, my magnetic band loosened some over the course of a run. I had no desire whatsoever to continue and try a paid app that might work slightly better than the ones I tried. I’ll stick with judging my vo2max based on my walking and running endurance, nothing else. Well I occasionally get to notice that running up a few flights of stairs causes no shortness of breath. So much for the stupid apple fitness watch.
Thanks for sharing your experience, George. I completely understand your frustration. That’s one of the reasons I wrote this article—Apple Watch VO₂ max is an estimate and doesn’t always reflect real-world fitness. If your endurance and recovery consistently tell a different story, it’s worth looking at the bigger picture rather than a single score. I think the metric is more useful for tracking long-term trends than judging your overall cardiovascular fitness. Thanks again for sharing your perspective!
Thank you for your clear exposition of a number of factors that we need to consider in interpreting what the Apple Watch cardio fitness metric gives. I recently switched from the Fitbit I had used for years, which pretty consistently told me my cardio fitness was excellent for my age (the exception was one measure right after I had my hip replaced and missed some cardio sessions—surprised at how quickly it reflected that). I switched because Google, which bought Fitbit changed how it reported cardio fitness. Apple Watch’s measure started off showing I was above average, but then started dropping and dropping until now I’m below average. I’ve been so concerned I saw a cardiologist yesterday. He’s ordered clinical tests. However, when I told him I routinely do 20+ minutes of fast walking (at 3mph) on my treadmill set to 4% incline, he said he’d guess my VO2 MAX would be about 24. The Apple Watch says I’m 15.5. That’s a pretty huge discrepancy. I’m very disappointed in the Apple Watch. I’d think it could do better. The other thing that really bugs me is that the research shows that the Apple Watch is the least accurate in measuring sleep compared with the Fitbit and the Oura ring in relation to sleep lab measurements. I’m feeling like I’ve bought a very expensive, inferior device full of bells and whistles.
Hi, Andrea! Thank you for sharing this. It’s a good reminder not to treat the Apple Watch VO₂ max number as a clinical measurement. The “estimate” is calculated from metrics like heart rate, pace, movement and workout data rather than directly measuring oxygen consumption. From what I have found, it can also become inconsistent if you stop or slow down to catch your breath during a workout without pausing the workout on the Watch itself. A difference like 15.5 versus an estimated 24 is notable. Having said that, even your doctor’s 24 reading is an estimate, and while the Apple Watch can be a useful fitness tool, I’m glad you’re getting proper clinical testing to get a more accurate picture rather than relying solely on it.