Your smartwatch might be giving your workouts more credit than they deserve

If you own a smartwatch, you know the satisfaction that comes after finishing a strenuous workout, glancing at your wrist, and seeing just how many calories you’ve burned. If your aim is to be in a calorie deficit and lose weight, that figure on the watch holds a lot of importance. However, a new study suggests that your smartwatch might not be giving you accurate readings, especially if you have a higher body fat percentage.

Smartwatches have become increasingly common, not just because of their health and fitness tracking prowess, but also because of their convenience. I use mine during every workout, and while previous reports about inaccuracies have made me err on the side of caution when interpreting the numbers my watch throws out, I still find myself looking at them after every workout regardless. Considering The Conversation’s new report on smartwatch inaccuracies, I might have to change that habit.

Researchers at Florida International University, using a Fitbit Sense 2, Garmin Forerunner 955, Samsung Galaxy Watch 5, and an Apple Watch Series 8, found that the wearables substantially miscalculated how many calories users burn during specific workouts.

For the study, the researchers recruited 58 adults and made them complete a standardized cycling session while wearing all of the above-mentioned devices at the same time (two on each wrist). As the subjects pedaled, the researchers measured their energy expenditure using a metabolic analyzer, which essentially captures and measures the gases a person inhales and exhales during exercise via a mask.

This is the most reliable way to measure calorie expenditure during a workout, and it gave the researchers an accurate figure to compare each smartwatch’s calorie estimates against.

The result? “All four watches demonstrated substantial error in measuring caloric expenditure,” reads the report.

Apple, Garmin, and Samsung reportedly all overestimated calorie expenditure. Apple was the least inaccurate of the bunch. Fitbit, on the other hand, produced data that wasn’t systematically positive or negative when compared to the median, but it occasionally produced wildly inaccurate results, including seven readings that exceeded the metabolic analyzer’s reading by 450%. These were treated as device errors.

That, however, wasn’t the most interesting observation. Researchers found that errors increased based on the participants’ body fat percentage. Essentially, the higher a person’s body fat percentage, the greater the error in calorie estimates. This was observed across all the tested wearables.

Errors ranged from 50% to 100%, and in some cases even more. So, for example, if the metabolic analyzer found an exercise to result in an energy expenditure of 100 calories, the wearables, when used by someone with a high body fat percentage, estimated calorie expenditures of 150 (50% more) and 200 (100% more).

This is particularly concerning, especially considering that people with higher body fat percentages may be among those most likely to rely on calorie expenditure estimates to track their weight-loss progress. According to the report, someone with a body fat percentage of 35% or higher who exercises for four hours in a week might be overestimating their calorie expenditure by roughly 1,200 to 2,400 calories.

The researchers couldn’t pinpoint why higher body fat percentages resulted in greater inaccuracies, especially since manufacturers keep their calorie-estimation algorithms proprietary. Users should take their wearable’s calorie estimates as just that: estimates.