Why You Can't Trust Your Fitness Tracker on Calorie Burn

Photo: Dapetrus (Shutterstock)

There was a time, before Fitbits, when nobody knew quite how many calories they were burning on a daily basis. Sure, you could calculate a rough estimate based on your body size, sex, and age; and you could choose whether or not to believe the calorie readout on the treadmills and bikes at the gym. But the idea that a gadget could tell you what you personally burn on this entire, particular day was revolutionary. It was also wrong. It is still wrong.

How fitness trackers calculate calorie burn

Before we consider how accurate fitness trackers are, let’s look at what they actually do. Most trackers use accelerometers to figure out when your body is moving, and by how much. If you have a watch on your wrist, and the watch swings back and forth rhythmically while sort of bouncing up and down, your gadget guesses that you must be walking. If there is quicker bouncing and your wrist makes a smaller movement, you’re probably running.

This is the basic idea behind how trackers detect how many steps you’re taking. If you’ve paid attention to your step count, you already know some of the ways this can be inaccurate. If you’re shopping, for example, keeping your hand on the shopping cart handle may result in you not getting credit for the steps you’re taking. (A wearable that clips onto your torso would be more accurate, but manufacturers seem to be moving away from the clip-on type.)

Then there’s the heart rate sensor. Since your hands don’t always move predictably during exercise, it can be easier to just tell your watch that you’ll be cycling or doing yoga or whatever. The gadget then uses your heart rate to make an educated guess about how much work your body is doing.

Whatever the source of the data—heart rate, movements, or a combination—the gadget uses a formula to calculate how many calories it thinks you’re burning. Your age, weight, and sex may figure into this equation. So the fitness tracker doesn’t actually know how many calories you’re burning; instead, it’s calculating a number based on incomplete information.

Factors that can affect a fitness tracker’s accuracy

If we were robots, all built the same, all moving in predictable patterns, this approach might work. But humans are complicated, and technology often gets confused.

For example, you may get different step counts if you put a gadget on your right versus left wrist. And the optical heart rate sensors that a lot of trackers use are less accurate on dark skin.

These problems relate to the data that the trackers gather, but then there’s the question of how the algorithms put it all together to get the number they show you when they say how many calories you burned. The companies that make fitness trackers aren’t required to publish their algorithms or verify that their calorie counts are accurate. They can just put a device on the market and there you are, comparing wearables on shopping sites without any information about how accurate they are, besides the companies’ claims.

Researchers are interested in fitness trackers’ accuracy, which would seem like a good thing. They want to be able to use wearables in research or recommend them for individuals and healthcare providers.

But there’s a huge delay in actually getting that information, and it’s often published too late to be useful. By the time a researcher buys a batch of the latest model, runs their study, writes it up, submits it to a journal, and finally gets it published, several years may have gone by and the company has moved on to the next model.

With that caveat, I still think it’s useful to look at some of the research on fitness trackers, to see what kinds of themes emerge. Are any of them good at estimating your calorie burn?

What studies say about fitness trackers’ accuracy

Alright, time for the bad news. A study from 2020, which looked at a variety of gadgets including Apple, Garmin, Polar, and Fitbit products, found that all the devices are inaccurate more often than they are accurate. The authors considered a device to be accurate if its reading was plus or minus 3% when compared to a more accurate measure of energy expenditure (that is, calorie burn) in a lab setting. Here’s how some of the top brands fared:

The fact that Fitbits were roughly correct on average doesn’t mean they were useful. If sometimes your device overestimates and sometimes it underestimates, it’s not very helpful unless you know which is which.

A 2018 review specifically of Fitbits found that accuracy varied greatly depending on factors like where they were worn (torso was more accurate than wrist), whether you were walking uphill, and whether you walked at a constant speed or stopped and started. The accuracy also varied by device, with the Fitbit Classic underestimating calorie burn and the Fitbit Charge usually overestimating. The devices just aren’t accurate enough to know how many calories you’re really burning

A more recent study, published earlier this year, compared the Apple Watch 6, the Fitbit Sense, and the Polar Vantage V. The researchers had volunteers wear all three gadgets while sitting quietly, walking, running, cycling, and strength training. Every gadget, for every activity, was awarded a judgment of “poor accuracy” with coefficients of variation ranging from 15% to 30%.

If these devices are all inaccurate, how am I supposed to know how many calories I’m burning?

It’s probably most useful if you think of your calorie burn as a number you cannot measure directly. Treat it as a black box: I burn some unknowable number of calories, now what?

The only common reason you would need an accurate estimate of calorie burn is if you are trying to figure out how much food you need to eat. If you want to lose weight, you want to eat less than you burn; if you want to gain weight, you want the reverse; and if you’re trying to make sure you maintain your weight, you want to eat roughly the same as what you burn.

The cool thing is that you can adjust how much you eat based directly on your weight, rather than using calorie burn estimates as a middleman. Let’s say you’re training for a marathon and you want to make sure you fuel yourself appropriately. Well, if you’re under-eating, you’ll start to lose weight. When you start to see the scale trend downward, that’s your signal to add a few hundred calories to your diet. If, after that adjustment, your weight stays steady, then you know you’re eating the right amount. As you increase your training (or if you take time off to rest a sprained ankle), you can make more adjustments as you go.

We have a post here detailing how to make these adjustments with the help of either a paid app, a group of free apps, or a DIY spreadsheet. If you’ve been using a fitness tracker instead, and it’s working for you, feel free to keep using it. But if the tracker ever stops giving you the results you want, go ahead and leave it out of the equation.

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