Apple Watch Series 12 VO2 Max is too low and needs calibration

HarryPotter177 wrote:

My Apple Watch Series 8 regularly recorded my vox2 max at 28 29 or 30. I walk daily, and lift/resistant bands

VO2 MAX is a measure of the maximum rate of oxygen volume your body can consume and utilize (ml/kg/min). The better one's cardio fitness, the higher this capacity will be. VO2 MAX is measured on a treadmill or stationary bike with a mask to track oxygen consumption rates starting from a base level and ramping up the level of effort until the maximum level is reached. At that maximum level, the heart rate is close to 90% or 95% of the maximum for one's age and the subject will reach the point of being close to passing out. I have done this on a treadmill in a clinic and it's an exhausting (and unpleasant) process to properly measure VO2 MAX.

A smart watch tries to approximate this using ONLY the user's pulse (heart rate) and speed (pace = distance divided by time) during an outdoor walk or run. Depending on the individual, this approximation may or may not work well. The algorithm that tries to discern VO2 MAX from just those three simple metrics is better suited to some individuals' physiology than others. Over time, smart watch makers may refine the algorithm used, so changes may be seen from that refinement when the algorithm is updated.

If one is saying that the old algorithm was better than the new one, well, one needs to provide some evidence of that. Which no one here has so far. Everyone likes better fitness numbers but it is what it is.

There is nothing to "calibrate" for a smart watch calculation. It just takes heart rate, distance covered, and time and calculates an approximate VO2 MAX.

https://www.harvardhealth.org/healthy-aging-and-longevity/vo2-max-what-is-it-and-how-can-you-improve-it

To improve one's VO2 MAX, usually one has to embark on a focused training regimen that includes interval training and brief excursions to close to one's max heart rate, followed by recovery, then repeating. This is fairly intense so usually rest days are incorporated. Progress over a period of ~ weeks can be discerned. However individuals can easily see improvements in their cardio capacity just by repeating intense and sustained workouts and seeing how their times improve. I do a 32 mile bicycle ride in a 10-mile loop and I have seen my times improve from 2.0 hours to 1.75 hours as my fitness improves. As for the Apple watch VO2 MAX metric, I started daily bicycle rides back in December 2025 and the VO2 MAX reported from my Series 11 was ~ 30 one year ago. Now, a year later, it is 43, which is high for my age range (60+). I don't know if I believe the absolute number but I do believe the trend. That 43 number has stayed consistent since June 2026 through multiple watch OS updates and an upgrade (to 27).

Since I’ve been using upgraded to series 12, the VOX2 max is around 20. This is not a problem with my workouts. It’s a problem with this watch.

Since the watch is using ONLY heart rate, distance covered, and time, if there is a problem with the watch, it has to be one of those three things. They are easy to check, especially distance and time. Heart rate has to be measured accurately and continuously during the workout for the algorithm to work, and you can check that your pulse is being properly recorded as well. If not, you should revisit the fit and band you are using. When people say their VO2 MAX changed unreasonably after changing to a new model of watch, I always suspect that the new watch might not fit as well, perhaps due to the band, but also sometimes due to the watch size itself and how the different model fits to and sits on the user's arm/wrist. If the heart rate sensor is actually broken, that can be easily checked by an Apple Authorized Service Provider.

In the Health app I clearly see an immediate decline from one day to the next one. The decline started the first day wearing the new watch.

That might indicate inconsistencies in measurement of one of those three quantities.

this is not a health problem for me. This is a hardware, software problem that Apple has on this watch.

One can blame the messenger for news one doesn't like. But unless one of those three items (pulse, distance, time) is being measured wrongly, it's hard to point to the Apple watch here.

peterkthomas wrote:

VO 2 Max number is too low as I walk ~ 25 to 30 miles per week and do resistance training 2-3 times per week. Is there a way to calibrate it? I have a series 12 watch.

Have you had your VO2 Max measured in a maximal exertion treadmill test in a laboratory environment? That might be a "truth" measurement to compare the Apple watch data to.

https://pubmed.ncbi.nlm.nih.gov/40373042/ and other similar scholarly papers have examined the Apple watch VO2 Max accuracy compared to medical laboratory data. "Overall, Apple Watch underestimated VO2 max, with a mean difference of 6.07 mL/kg/min (95% CI 3.77-8.38)."

Apple also has a document on this topic. "[Apple watch] estimates of VO2 max are based on submaximal predictions of VO2 max rather than peak VO2 ..." So that means that the watch is making a projection of VO2 max rather than actually using a peak VO2 which would require exercising to exhaustion. So this adds some uncertainty and likely a small underestimation of the actual value.

https://www.apple.com/healthcare/docs/site/Using_Apple_Watch_to_Estimate_Cardio_Fitness_with_VO2_max.pdf

I don't know of a way to "calibrate" VO2 max on an Apple watch. Just accept it for what it is, an approximation that is pretty good usually but is not super accurate as it makes approximations and projections that are generally measured directly in a medical lab true VO2 max measurement that has the subject exercise to exhaustion. I have had "exercise to exhaustion" measurements done in a medical lab and they are not very pleasant (I felt like I would pass out) and probably are not recommended for people with certain medical conditions or for most people over the age of 50 or so. What the Apple watch does is a pretty good substitute but does not require levels of exertion that might be dangerous for some, at the expense of some accuracy, presumably.

I suggest that you use the Apple watch VO2 max for tracking relative trends over time, rather than use it for some accurate absolute measurement.

peterkthomas wrote:

Thanks for the detail - no I have not had a lab test

You (and others who may also be reporting this same issue) can try this test:

https://www.trainermetrics.com/fitness-assessment-calculations/vo2max-one-mile-walk/

Follow the directions carefully.

Compare to your watch results from your previous workouts with the watch.

Meanwhile you can download an iPhone app called InstantHeartRate and compare its measurement of your pulse to that of your watch. This heart rate is crucial to VO2 MAX, the measurements of distance covered and time elapsed can easily be checked with other devices.

Also important: make sure your weight and age and sex are all properly entered.

peterkthomas wrote:

Thanks for the detail - no I have not had a lab test

VO 2 Max number is too low as I walk ~ 25 to 30 miles per week and do resistance training 2-3 times per week. Is there a way to calibrate it? I have a series 12 watch.

VO2 Max is best improved by doing exercises that significantly elevate the heart rate and cause you to breathe quite hard, for extended periods. I don't know how intense those walks you do are, but the watch will measure how much the heart rate is elevated during periods of intense exertion and then how quickly the heart rate comes back down. Resting pulse that is relatively high, and a heart rate that takes a long time to recover after intense exercise, could be signs of low cardiac fitness.

My resting pulse is in the low 40s and my watch CLAIMS I have cardiac fitness (VO2 Max) that is on the border of "high" and "very high" for my age group. The VO2 Max tends to decrease with age. So your age range is a major factor. Other key factors include: Age, Sex, Weight, Height. Key to the VO2 Max measurement is the measurement of heart rate. The pulse has to be consistently measured via the skin sensors on the underside of the watch and you should be getting an increase in heart rate of ~ 30% or more during the workout to get a good "signal." For instance, when I start walking my pulse goes up from ~ 40-45 to something in the range of 60-100, which is plenty of "data" for the Apple watch algorithms to compute how well the physiology is responding to exertion. Most of my exercise comes from fairly intense cycling workouts, rides of 20-40 miles, but the cardiac fitness measurements come from 3 mile walks I take with my wife every other day or so. (The watch does not use the cycling data for cardiac fitness but estimates cardiac fitness from those walks)

If your walks are fairly "leisurely," they won't help much with cardiac fitness. Strength resistance training also does not directly improve cardiac fitness, although it can be beneficial in other ways.

Also key -- the watch has to be able to make accurate and continuous estimates of pulse during the walk or run workouts. If there are gaps, e.g. periods when heart rate is not detected, this can really throw the cardiac fitness result way off. Have you checked that there are no gaps in pulse measurements and do those pulse measurements look about right to you? They should be increasing significantly during your exercise and then recovering quickly after the exercise ends.

So that we might better understand your concern, what are your current “as displayed” vs. some “reasonably assumed” values ?

My Apple Watch Series 8 regularly recorded my vox2 max at 28 29 or 30. I walk daily, and lift/resistant bands

Since I’ve been using upgraded to series 12, the VOX2 max is around 20. This is not a problem with my workouts. It’s a problem with this watch. In the Health app I clearly see an immediate decline from one day to the next one. The decline started the first day wearing the new watch.

this is not a health problem for me. This is a hardware, software problem that Apple has on this watch.

First I’m NOT disputing your findings …

… but just trying to understand the use case. How does it really matter ?

Are you not simply comparing it to your own readings and trends over time ?

If your previous normal was “X” and your new normal is “Y” … how is the different new normal an issue ?

Perhaps the previous watch was reading too high ?

The readings are after-all calculated values based on certain assumptions.

“Calibrate” it against what? Do you have another instrument to compare it to?

18.4 and I have nothing to compare it to other then wondering why it’s so low considering the exercise I get each week

Thanks for the detail - no I have not had a lab test

Sorry I don’t have another instrument