A new study finds that consumer-grade fitness wearables are generally inaccurate when calculating calories burned. The devices overestimate how many calories people are burning, and the error grows the fatter the person wearing it. NPR’s Leila Fadel spoke with the study’s lead author, Jason Kostrna, an associate professor of exercise science at Florida International University.
The 15% to 25% Overestimate
In the study, Kostrna and his team found a typical error of about 15% to 25%. That number varies based on individuals’ body fat percentage, with people carrying more fat getting less accurate readings.
The study’s big finding is that the body fat percentage of an individual was correlated with an increased error in how many calories the watch said you were burning. People with higher body fat percentage were getting estimates that were less accurate, with the watch telling them they burned more calories than they actually did.
Kostrna explained the math to Fadel: “In our study, if you were in a high body fat percentage of 35% or 40%, you might look down and see 100 calories on your watch. And in reality, you might’ve only burned 50 or 60.”
Why the Error Happens
The watches are trying to do something complicated. They take user-entered info when you set up your watch — your age, your gender, your height and your weight — and use heart rate information, accelerometer data of how much your wrist is moving and in what movement pattern, to calculate calories burned. There is a high chance that mistakes are made along the way, and individuals with higher body fat percentage may not have that equation calibrated as well.
Heart Rate Was Accurate, Mostly
The study focused on heart rate and calories, not step counts. Fadel asked whether the devices also got heart rate wrong, and Kostrna said the answer was mixed. “While there was technically some significance for heart rate, for most purposes, for most people, heart rate was actually fairly accurate, within a beat or two.”
The heart rate data was the one bright spot in the findings. The calorie numbers were the problem.
Brand Performance
Each brand had significant error overall, according to the study. Apple technically performed better than most. The researchers were using an Apple Watch, and it performed the best. The Samsung and Garmin apps, watches had some errors.
Fitbit had some device characteristic errors that the researchers saw at a slightly higher rate, but its data looked the best in some of the data.
| Brand | Performance |
|---|---|
| Apple | Best overall, used in study |
| Samsung | Some errors |
| Garmin | Some errors |
| Fitbit | Some device-characteristic errors, but data looked best in some cases |
Kostrna’s summary of the brands was blunt: “Each brand did have significant error overall.”
What the Study Shows
The study’s findings are specific to the devices tested. The pattern they found — that the watches overstate calories burned and that the error grows with higher body fat percentage — is a key part of the research.
The study’s authors say the error is systematic. The watches are not simply wrong by chance. The formula they use to estimate calories burned is miscalibrated for people with higher body fat, and the devices cannot know that on their own. Each watch relies on the same set of inputs — age, gender, height, weight, heart rate, wrist motion — and the math it does on top is where the error creeps in.
What Wearable Users Should Do
Kostrna’s advice is simple and conservative. When you look at the watches, estimates that they’re giving you, you want to be conservative, especially if you’re someone at a higher body fat percentage.
“So when you look at the watches, estimates that they’re giving you, you want to be conservative, especially if you’re someone at a higher body fat percentage.”
The study does not tell you to treat the numbers as gospel. It tells you to read them with some doubt.
The Takeaway
The error is not trivial. A 15% to 25% overestimate means a person could be off by a significant margin.
The study does not prove that all watches are useless. It finds that the devices tested overestimate calories burned, and the accuracy gaps vary by brand and by the wearer’s body fat percentage.
Source material: “Your smartwatch isn't accurately counting how many calories you burn, study says,” NPR.
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