There was a time, before Fitbits, when no one knew exactly how many calories they burned each day. Sure, you could work out a rough estimate based on your body size, gender, and age; and you could choose whether or not to believe in calorie readings on treadmills and bikes at the gym. But the idea that a device can tell you what you personally burn all this particular day it was revolutionary. He was bad too. It’s still wrong.
How Fitness Trackers Calculate Calorie Burn
Before we consider 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 the watch rocks back and forth rhythmically as it bounces up and down, your device guesses that you must be walking. If there is a faster bounce and your wrist makes a smaller movement, you are probably running.
This is the basic idea behind how trackers detect how many steps you are taking. If you’ve been paying attention to your step count, you already know some of the ways this can be inaccurate. If you’re out shopping, for example, keeping your hand on the shopping cart handle can result in you not getting credit for the steps you’re taking. (A wearable device that clips onto the 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 in a predictable way during exercise, it can be easier to tell your watch that you’re going to ride a bike or do yoga or whatever. The device then uses your heart rate to make an educated guess about how much work your body is doing.
Regardless of the source of the data — heart rate, movement, or a combination — the device uses a formula to calculate how many calories it thinks you’re burning. Your age, weight, and gender can all figure into this equation. So the fitness tracker doesn’t really to know how many calories are you burning; instead, you’re calculating a number based on incomplete information.
Factors that can affect the accuracy of a fitness tracker
If we were robots, all built the same way, all moving in predictable patterns, this approach might work. But humans are complicated, and technology often gets confused.
For example, you can get different step counts if you put a device on your right or left wrist. And the optical heart rate sensors that many trackers use are less accurate on dark skin.
These issues relate to the data the trackers collect, but then there’s the question of how the algorithms put it all together to come up with the number they show when they say how many calories you burned. Companies that make fitness trackers are not required to publish their algorithms or verify that their calorie counts are accurate. They can simply put a device on the market and there it is, comparing wearable devices on shopping sites without any information on how accurate they are other than the companies’ claims.
Researchers are interested in the accuracy of fitness trackers, which seems like a good thing. They want to be able to use wearable devices in research or recommend them for individuals and health care providers.
But there is a long delay in getting that information, and it is 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 publishes it, several years may have passed 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 come up. Are none of them good for estimating your calorie burn?
What studies say about the accuracy of fitness trackers
All right, time for the bad news. A study from 2020, which analyzed a variety of devices 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 within plus or minus 3% of a more accurate measure of energy expenditure (ie, calorie burn) in a laboratory setting. Here’s how some of the top brands fared:
- Garmins underestimated calorie burn 69% of the time.
- Apple Watches overestimate calorie burn 58% of the time.
- Polar devices overestimated calorie burn 69% of the time.
- Fitbits underestimated 48% of the time and overestimated 39% of the time.
The fact that the Fitbits were more or less correct on average doesn’t mean they were useful. If your device sometimes overestimates and sometimes underestimates, it’s not very useful unless you know which is which.
A 2018 review specifically from Fitbits he found that accuracy varied greatly depending on factors such as where they were worn (the torso was more accurate than the wrist), whether he was walking uphill, and whether he was walking at a constant speed or stopping and going. Accuracy also varied by device, with Fitbit Classic underestimating calorie burn and Fitbit Charge generally overestimating. The devices just aren’t accurate enough to know how many calories you’re actually burning.
A most 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 devices while sitting quietly, walking, running, cycling, and doing weight training. strength. Each device, for each activity, was given a “poor accuracy” rating 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?
It’s probably more helpful if you think of your calorie burn as a number that you can’t measure directly. Treat it like a black box: I burn some unknown number calories, now what?
The only common reason you’d 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 want eat less than you burn; if you want to gain weight, you want the opposite; and if you’re trying to make sure you maintain your weight, you’ll want to eat about as much as you burn.
The good thing is that you can adjust the amount you eat according to 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 make sure you’re fueled properly. Well, if you are eating little, you will start to lose weight. When you start to see the scale trending downward, that’s your cue to add a few hundred calories to your diet. If, after that adjustment, your weight stays stable, then you know you’re eating the right amount. As your training increases (or if you take time off to rest a sprained ankle), you can make further adjustments as you go.
We have a post here detailing how to make these adjustments with the help of a paid app, a bunch of free apps, or a DIY spreadsheet. If you have been using a fitness tracker and it works for you, feel free to keep using it. But if the tracker stops giving you the results you want, go ahead and leave it out of the equation.
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