Your Fitness Tracker Has No Idea How Many Calories You Burn
This post is part of Find Your Fit Tech , a fitness wearable buying guide from Lifehacker. I ask tough questions about whether wearables can really improve your health, how to find the one that’s right for you, and how to make the most of the data wearables have to offer.
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. You can choose whether or not to believe the calorie readings on treadmills and bikes at the gym. But the idea that a gadget on your wrist could tell you how many calories you personally burned in one particular day was revolutionary. This was also wrong. Even though wearable fitness devices have become more advanced, this is still wrong.
How fitness trackers calculate calorie burn
Before I talk about how accurate fitness trackers are, let’s look at what they actually do. Most trackers use accelerometers to detect when your body is moving and how much. If you have a watch on your wrist and it swings rhythmically back and forth, as if bouncing up and down, your gadget knows that you must be walking. If the jumping is faster and your wrist makes less movement, you are probably running.
This is the basic idea of how trackers determine how many steps you take. If you’ve been paying attention to the step count, you already know why it might not be accurate. For example, if you are shopping, keeping your hand on the handle of a shopping cart may result in you not being credited for your actions. (Wearables that attach to the torso would be more accurate, but manufacturers seem to be moving away from clip-on devices.)
There’s also a heart rate sensor: Since your arms don’t always move predictably during a workout, it might be easier to just tell the watch 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.
Regardless of the data source—heart rate, movement, or a combination of both—the gadget uses a formula to calculate the number of calories it thinks you’re burning. This equation may include your age, weight, and gender. However, in general, a fitness tracker doesn’t actually know how many calories you’re burning; instead, it calculates a probable number based on incomplete information.
Factors that may affect the accuracy of a fitness tracker
If humans were robots, built the same and moving in predictable patterns, this formulaic approach might work. But people are complex, and technology is often confusing.
For example, you may get a different number of steps if you place the gadget on your right and left wrist . And the optical heart rate sensors that many trackers use may be less accurate on dark skin .
These problems stem from the data the trackers collect, but the question is how the algorithms put it all together to come up with the number they show you when they tell you how many calories you’ve burned. Fitness tracker companies are not required to publish their algorithms or verify the accuracy of calorie counts. They can just release the device to the market and here you are comparing wearables on shopping sites without any information about how accurate they are other than what the companies say.
Researchers are interested in the accuracy of fitness trackers, and this would seem 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 getting this information, and it is often published too late to be useful. By the time a researcher buys a batch of the latest model, conducts the research, writes it up, submits it to a journal, and finally publishes it, several years may have passed before the company has moved on to the next model. .
With that caveat, I still think it’s useful to look at the research on fitness trackers to see what themes emerge. Do any of them know how to estimate your calorie expenditure?
What Research Says About Fitness Tracker Accuracy
Time for some bad news. A 2020 study that looked at various gadgets from Apple, Garmin, Polar and Fitbit found that all devices were more often inaccurate than accurate. The authors considered a device accurate if its readings were within plus or minus 3% of a more reliable measure of energy expenditure (i.e., calorie burn) in a laboratory setting. Here’s how some of the leading brands are faring:
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Garmins underestimated calorie expenditure 69% of the time.
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Apple sees overestimation of calorie burn 58% of the time.
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Polar devices overestimated caloric expenditure 69% of the time.
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Fitbits underestimated calorie burn 48% of the time and overestimated it 39% of the time.
Just because Fitbits were about right on average doesn’t mean they were helpful. If your device sometimes overestimates and sometimes underestimates, it won’t help much if you don’t know which is which.
A 2018 review of Fitbits found that accuracy varied greatly depending on factors such as where you wore them (the torso was more accurate than the wrist), whether you walked uphill, walked at a constant speed, or stopped and started. . Accuracy also varied by device, with the Fitbit Classic underestimating calories burned while the Fitbit Charge tending to overestimate. The devices are not accurate enough to determine how many calories you actually burn.
A more recent study , published in 2022, compared the Apple Watch 6, Fitbit Sense, and Polar Vantage V. Researchers asked volunteers to wear all three devices while they sat quietly, walked, ran, biked, and strength-trained. Each gadget for each activity was rated as “poor accuracy” with coefficients of variation ranging from 15% to 30%.
If all these devices are inaccurate, how can you know how many calories you are burning?
It’s probably most helpful to think of calories burned as a number that can’t be measured directly. Think of it as a black box: I burn an unknown amount of calories, now what?
The only common reason you might need an accurate estimate of your calorie burn is to try to figure out how much food you need to eat. If you want to lose weight, you need to eat less than you burn ; if you want to gain weight, you need the opposite; and if you’re trying to maintain your weight, you need to eat about the same amount as you burn.
The best part is that you can adjust how much you eat directly based on your weight, rather than using calorie burn estimates as a proxy. Let’s say you’re training for a marathon and want to make sure you’re eating right. Well, if you are undernourished, you will start to lose weight. When you start to see the scale trending lower, that’s a sign that you should 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 intensity of your workouts (or if you take time off to rest a sprained ankle), you can make more adjustments as you go.
I have a post that details how to make these changes using a paid app, a group of free apps, or a DIY spreadsheet. If you’ve been using a fitness tracker instead and it works for you, feel free to continue using it. But if the tracker ever stops giving you the results you want, you can safely remove it from the equation.