5 Health Metrics Your Smartwatch Is Probably Measuring Inaccurately

Part of the overall appeal of wearing a smartwatch, aside from the quick access to essential apps from your Android or iPhone, is the various health tracking capabilities. Smartwatches, at least on paper, can provide information on a variety of health metrics, including your calorie burn, heart rate, sleep tracking, and VO₂ Max, which can be invaluable both during workouts and as individual metrics. We say “on paper” because, unfortunately, while smartwatches can collect a lot of data during regular use, knowing whether it’s 100% accurate is another matter.

Smartwatches aren’t magic; they can’t just feel everything that’s going on in your body. The health data collected is based on several observable conditions like your pulse or arm movements, and more often than not, using it to extrapolate certain types of information results in more of an educated guess than an absolute fact. This can be helpful for developing good health habits, but it’s certainly no replacement for accurate metabolic data, and treating it as such could be bad for your overall health in the long run.

One of the most basic health metrics that smartwatches can track is how many steps you took in a given day. Step tracking isn’t rocket science; After all, basic pedometers have been around for hundreds of years. Even so, it’s obviously wrong to measure the number of steps you take with something that’s attached to your arm rather than your leg.

You could try wearing an Apple Watch on your ankle, but that would pose its own problems. A smartwatch’s pedometer tends to underestimate the actual number of steps you take by 5-10%. It’s not huge, but if you’re going a long distance, it could mean hundreds of steps aren’t measured. More to the point, even these ideal conditions depend on your precise arm pumping based on the number of steps you take.

When you use a smartwatch to track steps when you lift or push objects, for example, it won’t give an accurate reading because your arms likely aren’t moving at the same rate or rhythm as your legs. Step counts are great for joggers and runners, but they’re a much less useful health metric for casually going about your business.

When it comes to biometrics, heart rate is generally easier to measure. Technically, you don’t need a smartwatch for this; simply check your pulse during exercise by holding a finger at the top of your neck. Still, getting a quick heart rate reading is handy if you’re doing high-intensity exercise, and for the most part, smartwatches are pretty good at detecting it. Of course, “good enough” is not the same as “perfect.”

Smartwatches typically use optical heart rate sensors, shining a light on your wrist and measuring blood flow in your veins. This is a fairly reliable way to measure heart rate, with an error rate of around 5%. However, this error rate only applies during low intensity exercises. The more vigorously you move, the harder it becomes for this optical sensor to get a consistent reading of your blood flow, hampering estimation. Additionally, factors such as sweat and skin tone can interfere with the sensor, meaning some people may receive naturally less accurate readings. This isn’t a major problem, although it can be problematic for an exercise routine if you rely on specific heart rate zones.

Some smartwatches claim to be able to track the amount of calories you burn when you exercise or perform strenuous tasks. If you’re dieting, it can definitely be helpful to know how much energy you’re burning so you know how much food to eat. However, a smartwatch can’t actually track calorie expenditure, it simply estimates this number based on a few factors such as heart rate and movement.

It would take an extremely advanced algorithm for a smartwatch to calculate this measurement accurately, and current smartwatches simply aren’t capable of that. As a result, these wearable devices are typically incorrect on calorie burn estimates by at least 20%, plus or minus. If you take your smartwatch’s calorie consumption stats as gospel, you may end up eating more than you planned to compensate, or less than you need if you think you’re overindulging. Either way, these are not healthy habits.

Another popular application for smartwatches is to track your nighttime sleep quality. By wearing a smartwatch to bed, the device can measure how much you toss and turn to determine when you enter different sleep states and for how long, providing an overall score of your sleep quality in the morning. But these wearable devices can’t measure the brain activity needed for a 100% accurate reading, so they have to take shortcuts, which is why tracking sleep with a smartwatch may not really help.

Smartwatches generally summarize your sleep quality by combining motion detection and heart rate measurement. This method can detect when you enter and exit sleep, but as for detecting different stages of sleep, it has no idea. When it comes to detecting changes in sleep stages, smartwatches only have about a 50-65% success rate, a real headache. Rather than tracking your sleep quality, it’s better to rely solely on smartwatches to determine your sleep duration.

A great way to estimate exercise intensity is to measure the maximum volume of oxygen in your body during a workout, also known as VO₂ max. Determining your personal VO₂ max index will tell you how much oxygen your body is able to use during intense physical activity. Oxygen is any human’s most vital resource, and some smartwatches claim to be able to measure VO₂ max to help you determine your metaphorical fitness ceiling. However, a smartwatch can’t actually measure the amount of oxygen flowing into and out of your lungs.

You would need a complete medical lab setup for this. This statistic is just an educated guess on the part of the watch, estimated from movement and heart rate. It will assume that if you can move faster with the same heart rate as walking, you have a high VO₂ max. This often leads to overestimated values ​​for less active people and underestimated values ​​for those who are more active. Seeing this number increase over the course of a long workout can be a good motivator to improve your lung capacity, but it shouldn’t be treated the same as a lab-quality reading.