Fitness Trackers Consistently Misestimate Calorie Burn, Studies Find
Research shows fitness trackers are inaccurate on calorie burn more often than not. Here's what the studies found and what to do instead.
When testing smartwatches and fitness trackers, it’s reasonable to place particular emphasis on accuracy: Is the running pace correct? Does the device capture the ups and downs of heart rate? Some reviewers even get VO2max lab tests to check a bunch of watches’ fitness scores. But one thing that rarely gets tested for accuracy is calorie burn. Devices routinely disagree on the number of calories they think a user is burning - and 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.
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. You could choose whether or not to believe the calorie readout on the cardio machines at the gym. (You shouldn’t.) But the idea that a gadget on your wrist could tell you how many calories you personally burned during one particular day was revolutionary. It was also wrong.
How Fitness Trackers Calculate Calorie Burn
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.
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, though this depends on the device.
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 processes it through a formula to calculate how many calories it thinks you’re burning. Your age, weight, and sex may figure into this equation. Importantly, though, the fitness tracker doesn’t actually know how many calories you’re burning; instead, it’s calculating a probable number based on incomplete information.
Why There’s No Simple Test for Calorie Burn 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. For example, you may get different step counts if you put a device on your right versus left wrist. And the optical heart rate sensors that a lot of trackers use may be less accurate on dark skin compared to lighter skin.
These problems relate to the data that the trackers gather, but calorie burn isn’t a direct measurement. It’s an estimate derived from doing math with the data the watch measures, and different algorithms can come up with different calorie burn numbers depending on how the algorithm is designed. 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.
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, and so they’ll run studies comparing consumer devices to lab equipment. 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 a 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.
A 2025 review found that 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 of that review, the Apple Watch was already up to the Series 12.
What Studies Say About Fitness Trackers’ Accuracy
A study from 2020 that looked at a variety of gadgets from Apple, Garmin, Polar, and Fitbit 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 reliable measure of energy expenditure (that is, calorie burn) in a lab setting. Here’s how some of the top brands fared:
- Garmin devices underestimated calorie burn 69% of the time.
- Apple watches overestimated calorie burn 58% of the time.
- Polar devices overestimated calorie burn 69% of the time.
- Fitbits underestimated calorie burn 48% of the time and overestimated 39% 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 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.
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%.
The 2025 review of Apple Watch studies found that calorie burn was, on average, off by about 18%. The authors mention a proposed standard from the International Electrotechnical Commission that recommends fitness trackers not be off by more than 10%. That’s clearly not being met, although the researchers noted that newer models may be more accurate than earlier ones.
A 2026 study looked specifically at how accuracy relates to skin tone and to the amount of fat a person has. Skin tone didn’t seem to affect calorie counts very much, but adiposity (fat content) did. The higher a volunteer’s body fat percentage was, the more inaccurate their calorie burn result. The Garmin Forerunner 955 and Samsung Galaxy Watch 5 seriously overestimated calories; the Apple Watch Series 8 also overestimated but not as much; and the Fitbit Sense 2 underestimated when it wasn’t producing wild outliers. The authors concluded that all devices “substantially misestimate” calorie burn during exercise.
To get a sense of these percentages: if your true calorie burn is 2,000 calories per day, a device that’s off by 15% - on the low end of what these studies found - 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.
What to Do If You Can’t Trust the Number
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 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. It’s considered more accurate to adjust how much to 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. If you’re under-eating, you’ll start to lose weight. When you start to see the scale trending 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.
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.