Smartwatches and fitness trackers are generally pretty great, as they’re packed with plenty of features that promise to offer a solid overview on your health.
Among the likes of heart rate monitoring and workout tracking, many of the best smartwatches and best fitness trackers provide users with a so-called fitness age which has the power to either give you bragging rights or keep you incredibly humble.
Having said that, how much weight does a device’s calculated fitness age really have? Should you take the age with a pinch of salt, or is there factual evidence behind the number on the screen?
I’ve explored the science behind the so-called fitness age and found out whether you really need to worry about it.
How does a smartwatch calculate fitness age?
The exact measurements that go into a smartwatch’s fitness age calculation can vary slightly from device to device. However, calculating fitness age ultimately comes down to your VO2 max measurement, which is the maximum volume of oxygen that your body can use per minute. The higher the figure the better, as this means your body is generally able to take in more oxygen during more strenuous workouts.
With this in mind, it’s unsurprising that Garmin assesses your VO2 max figure to accurately decipher your fitness age. Garmin explains it does this by comparing your figure to those of people “of different ages within your same gender”.
However, Garmin states that on its newer devices, fitness age takes additional factors into account. These factors include activity intensity, your resting heart rate and, controversially, your BMI.
The latter is controversial, as many have criticised BMI as being an outdated way of measuring someone’s actual health. In fact, a 2023 study revealed that BMI “does not properly assess body fat percentage and muscle mass or distinguish abdominal fat from gluteofemoral fat” and explains that women and minorities were either “not included at all, or only comprised a small percentage of the 7462 men” within a 1972 study which sparked the modern use of BMI in research and medical settings.
Alongside VO2 Max levels, Whoop analyses your resting heart rate, sleep pattern and the amount of time each week you’ve spent exercising to determine your Whoop age. In addition, Whoop also factors in your lean body mass, if you’ve entered that detail into the app.
Otherwise, devices like Apple Watch and Fitbit don’t actually offer a built-in fitness age. Instead, you can download third-party apps for your Apple Watch which measure your potential fitness age, while Fitbit offers a cardiovascular fitness score rather than an age.
What is VO2 max, and do wearables provide an accurate measurement?
So, although smartwatches assess multiple factors, your individual VO2 max level is one of the key measurements to determine your fitness age – but how do devices actually measure this?
Both Garmin and Whoop disclaim that their VO2 max level is an estimate and isn’t totally accurate. Even so, both do provide insights into how they reach their respective figures. Whoop, for example, explains it estimates VO2 max by using its own three-tier proprietary algorithm that analyses your health and fitness data and “your demographic data including your age and biological sex”. Whoop promises that its VO2 Max tracking is within 3.3 to 3.7mL/kg/min of a lab-measured value. That roughly translates to around 7 to 8% of error.
Garmin, on the other hand, explains that its VO2 max estimations rely on an accurate user profile and your elevated heart rate. While Garmin doesn’t publicly reveal its level of accuracy, I’ve seen various reports of anywhere between 5 to 15% of error. However, a study conducted back in 2024 determined the accuracy of VO2 max estimates specifically with the Garmin Forerunner 265, which suggested that Garmin “may overestimate oxygen consumption compared to values acquired in the laboratory”.
Garmin VO2 max
Whoop VO2 max
I have a Whoop MG and a Garmin Cirqa, so had a look to see how each device has determined my VO2 max level. Interestingly, they differ slightly with their exact measures, with Whoop claiming my VO2 max is 46 while the Cirqa states a result of 43. Though Whoop’s estimate is higher, my fitness age is slightly older than Garmin’s at a very precise 28.9 years old. Garmin, ever the flatterer, says I have the fitness of a 26 year old.
Although the results aren’t too dissimilar from one another, the fact they vary slightly confirms that VO2 max and fitness age shouldn’t be taken as gospel, as two devices I wear constantly have provided me with different results.
Verdict – does fitness age matter?
Considering VO2 max is one of the key factors in determining an individual’s long-term health, the fact that Whoop and many of the best Garmin watches have the ability to estimate your level is undeniably brilliant. Sure, you can’t expect a 100% accurate reading, but an approach is surely better than nothing, right?
However, that error margin should be factored in when receiving your estimate, and therefore your fitness age, as it means the levels are likely not completely accurate. They might not be too far off, with Whoop promising up to 8% of error, but that’s still enough doubt that you shouldn’t completely rely on the ages provided. Not only that, but remember devices factor in additional measurements that are seen as outdated in modern terms, like BMI.
Essentially, we’d advise that while your fitness age is a solid indicator of your overall fitness level, it should be taken with a pinch of salt. That’s not to say you should ignore an age that’s double your actual age, nor that you shouldn’t be pleased with one that’s younger, but just keep in mind that it isn’t actually a completely reliable measurement.
