When I test smartwatches and fitness trackers, I always pay attention to accuracy. Is my running pace correct? Does the device capture the ups and downs of my heart rate? I even got a VO2max lab test to check a bunch of watches' fitness scores. But you'll notice that one thing I don't test for accuracy is calorie burn. None of my devices come close to agreeing on the number of calories they think I'm burning, and I don't expect them to. Even scientists who study the accuracy of wearables can't answer the question in a way that's useful when you're shopping for this year's devices—but we'll get into why that is below.
Before I discuss how accurate fitness trackers are, let’s look at where they get their numbers. For calorie calculations, the main sources are motion and heart rate data. For motion, 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.
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.
Why there's no simple test for accuracy
If humans were robots, all built the same, all moving in predictable patterns, this formulaic approach might work. But humans are complicated, and technology often gets confused.
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, outside of the companies’ claims.
That delay is why I (usually) can't use scientific studies to weigh in on the devices I write about. Here's a great example of how frustrating it can be: this review was published in 2025, and found the Series 1 was the Apple Watch that turned up the most often in the studies the authors were able to gather. The Series 9 and 10 watches were completely missing from the available data, and as a reminder, we're now up to a Series 11. With that caveat about delays, I still think it’s useful to look at the research on fitness trackers to see what themes emerge. Are any of them good at estimating your calorie burn?
Garmins underestimated calorie burn 69% of the time.
Polar devices overestimated calorie burn 69% of the time.
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 2022 study 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%.
To get a sense of these percentages, let's say your true calorie burn is 2,000 calories per day. A device that's off by 15% might report that you burned 1,700 calories, or that you burned 2,300. If you're using your device to figure out how much to eat, you could be way off in meeting your calorie goals.
If these devices are all inaccurate, how can you know how many calories you're burning?
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 maintain your weight, you want to eat roughly the same as what you burn. But think about it this way: you don't actually need to know your calorie burn if you have the other two terms in the equation—your calorie intake, and your weight.
I 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, you can safely leave it out of the equation.
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