If you wear a fitness tracker you probably rely on the data it gives you. You may meticulously track how well you slept, how many steps you took, your heart rate and probably how many calories you burned. But a study shows the calorie count may not be as accurate as you think.

Fitness trackers aren’t perfect. And though their technology has improved since they first came on the scene, they’re better at tracking some things than other. The reason they have such a hard time with calories burned has a lot to do with how differently your body burns calories compared to someone else’s body.

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Researchers at Stanford University and the Swedish School of Sport and Health Sciences found most trackers had the hardest time measuring energy expenditure. That just means a number of calories your body burns throughout the day and during exercise.

The study, published Wednesday in the Journal of Personalized Medicine, looked at eight direct to consumer trackers from as many manufacturers, including the Apple Watch and the Fitbit Surge. There were 60 participants 18 years of age and older in the study who completed 80 tests while using the trackers. The group was made up of 29 men and 31 women who were healthy and represented a wide variety in age, height, weight, body mass index, wrist circumference and fitness level, the study said.

The activities that included running, walking and cycling resulted in heart rate levels were reported with an acceptable margin of error. But none of the wrist-worn trackers measured or reported energy expenditure, or calories burned, within an acceptable margin of error. Of the eight devices, the Apple Watch had the smallest range of error while the Samsung Gear S2 had the biggest.

The trackers aren’t regulated by the government because they aren’t meant to be used as a source of reliable medical data. Even so, that doesn’t stop some insurance companies from using the data to provide wearers with discounts.

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The reason the energy expenditure data may be so off has to do with the individuality of users. While the fitness trackers themselves are physically one size fits all in many cases, their software isn’t necessarily. Some people have faster metabolisms meaning they burn calories more quickly and can burn more during a day than others. Additionally, certain exercises can boost your metabolism so it’s not a static facet of the body. Factors like gender, age and weight can all contribute to how quickly you metabolize calories.

This could be one reason the trackers couldn’t accurately predict calories burned. “Some people are incredibly efficient and look incredibly elegant when they run. And others really clearly look like they're burning a lot more calories to cover the same amount of ground," Euan Ashley, a researcher involved in the study from Stanford told Scientific American.

So wearing that tracker won't hurt, but you may need to put in some extra work to find out how many calories you're really burning.