Why You Can’t Trust Your Fitness Tracker for Calorie Burning
There was a time, before Fitbits, when no one knew exactly how many calories they burned daily. Of course, you can calculate a rough estimate based on your body size, gender, and age; and you could choose whether or not to believe the calorie readings on the treadmills and bikes at the gym. But the idea that a gadget could tell you what you were personally burning for that particular day was revolutionary. This was also wrong. It’s still wrong.
How fitness trackers calculate calories burned
Before we look at how accurate fitness trackers are, let’s take a look at what they actually do. Most trackers use accelerometers to determine when your body is moving and how much. If you have a watch on your wrist, and it rhythmically swings back and forth, as if bouncing up and down, your gadget guesses that you must be walking. If you’re jumping faster and your wrist is making smaller movements, you’re probably running.
This is the basic idea of how trackers determine how many steps you take. If you’ve paid attention to step counting, you already know how inaccurate it can be. For example, if you’re shopping, if you keep your hand on the handle of a shopping cart, it could result in you not being credited for the steps you take. (A wearable device that clips to the torso would be more accurate, but manufacturers seem to be moving away from the clip-on type.)
Then the heart rate sensor. Since your hands don’t always move predictably during exercise, it might be easier to just tell the watch that you’ll be cycling, doing yoga, or whatever. The gadget then uses your heart rate to make an educated guess about how much work your body is doing.
Whether the data source is heart rate, movement, or a combination of both, the gadget uses a formula to calculate how many calories it thinks you’re burning. Your age, weight, and gender may all come into play in this equation. So the fitness tracker doesn’t really know how many calories you’re burning; instead, it calculates a number based on incomplete information.
Factors that can affect the accuracy of a fitness tracker
If we were robots, built in the same way and moving in a predictable pattern, this approach might work. But humans are complex, and technology often gets confused.
For example, you can get a different number of steps if you put the gadget on your right and left wrist . And the optical heart rate sensors that many trackers use are less accurate on dark skin .
These problems are related to the data that trackers collect, but the question is how the algorithms combine all this to get the number they show you when they tell you how many calories you have burned. Fitness tracker companies are not required to publish their algorithms or verify the accuracy of calorie counts. They can just put the device on the market, and here you are comparing wearables on shopping sites without any information about how accurate they are, other than the companies’ claims.
Researchers are interested in the accuracy of fitness trackers, which seems to be a good thing. They want to be able to use wearables in research or recommend them to individuals and healthcare providers.
But there is a huge delay in receiving this information, and it is often published too late to be useful. By the time a researcher buys a batch of the latest model, does their research, writes it down, submits it to a journal, and finally publishes it, it may be several years before the company moves on to the next model.
With that caveat, I still find it helpful to look at some of the fitness tracker research to see what themes come up. Are any of them good at estimating calories burned?
What Research Says About Fitness Tracker Accuracy
Okay, time for bad news. A 2020 study that looked at various gadgets including Apple, Garmin, Polar and Fitbit products found that all devices are more inaccurate than accurate. The authors considered a device to be accurate if its reading was plus or minus 3% compared to a more accurate measurement of energy expenditure (i.e., calorie burn) in the laboratory. Here’s how it’s doing for some of the top brands:
- Garmins understates calories burned 69% of the time.
- Apple Watch overestimated calories burned 58% of the time.
- Overrated Polar devices burn 69% of the time.
- Fitbits underestimated 48% of the time and overestimated 39% of the time.
Just because Fitbits were about right on average , doesn’t mean they were useful. If your device sometimes overrates and sometimes underrates, it’s not very helpful if you don’t know which is which.
A 2018 review of Fitbits found that accuracy varied greatly depending on factors such as where they were worn (the torso was more accurate than the wrist), whether you were going uphill and walking at a constant speed, or stopping and starting. . Accuracy also varied by device, with the Fitbit Classic underestimating calories burned and the Fitbit Charge typically overestimating. The devices are simply not accurate enough to know how many calories you are actually burning.
A more recent study published earlier this year compared the Apple Watch 6, Fitbit Sense, and Polar Vantage V. The researchers asked volunteers to wear all three gadgets while sitting quietly, walking, running, cycling, and strength training. Each gadget for each activity was assigned a “Poor Accuracy” score with coefficients of variation ranging from 15% to 30%.
If all these devices are inaccurate, how am I supposed to know how many calories I’m burning?
This is probably most useful if you think of calories burned as a number that you can’t measure directly. Treat it like a black box: I’m burning some incomprehensible amount of calories, what’s next?
The only common reason you might need an accurate calorie burn estimate is if you’re trying to figure out how much food you need to eat. If you want to lose weight, you must eat less than you burn ; if you want to gain weight, you want the opposite; and if you’re trying to make sure you’re maintaining your weight, you need to eat about as much as you burn.
The cool thing is that you can adjust the amount you eat based directly on your weight, rather than using calorie burn estimates as an intermediary. Let’s say you’re training for a marathon and want to eat right. Well, if you’re undernourished, you’ll start to lose weight. When you start to notice that the scale is trending down, that’s a signal to add a few hundred calories to your diet. If your weight remains stable after this adjustment, then you are eating the right amount. As you increase the number of workouts (or if you take a break to rest your sprained ankle), you can make more adjustments along the way.
We have a post detailing how to make these adjustments with 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, leave it in the equation.