How Accurate Is Your Fitness Tracker?

The bigger question may be: Does it matter?

The goal is 10,000 steps a day. That number, the aim of people who wear fitness trackers like Fitbits and Jawbones, comes from the American Heart Association. While these devices track our steps, they don’t keep an accurate count on how much energy we burn.

Scientists looked at the Fitbit Flex and Jawbone Up24, popular fitness trackers, and found that these gadgets didn’t provide precise accounts of how much energy we burn. (In fact, Fitbit is facing a class-action lawsuit over the accuracy of its heart rate monitors.) The researchers worked with a controlled group of people wearing fitness trackers, closely monitoring every calorie the subjects consumed and burned. They also asked the control group to drink water treated with a chemical that makes it easy to detect energy output with a urine test. 

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Researchers found the fitness tracker measurements under and over estimating energy expended. The low counts were as much as 278 calories. The over estimations were as high as 204 calories. A second experiment showed different results with trackers ranging from 69 to 590 calories lower than the urine tests. 

“The results are troubling because when fitness trackers overestimate exercise, people who need more exercise to maintain or lose weight might get too little activity, increasing their risk for obesity and other chronic health problems,” said Motohiko Miyachi of the National Institute of Health and Nutrition in Tokyo and senior author of the study.

At the same time, underestimating exercise is just as risky. Someone with heart disease, for example, may push too hard if the numbers on the device are too low.

Tell that to people who are devoted to their fitness trackers, and they’ll argue that the control group had only nine men and 10 women. “That’s an awfully small group to test the accuracy of these devices,” said Craig Campoli, a devoted Fitbit wearer. 

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The study participants were between the ages of 21 and 50. They tested 12 different devices. In the first study, the researchers measured carbon dioxide production and oxygen consumption to assess each person’s energy expenditure. They observed how many calories the subjects consumed and how much energy they used over a 24-hour period. Half of the devices underestimated energy used, and the rest overestimated it. 

The second study lasted longer; participants wore devices for 15 days, and researchers collected urine samples on the eighth day. In this study, every fitness tracker underestimated energy expended.

The scientists noted that it was possible that some participants removed their devices when bathing or recharging batteries; they said that might be one reason the devices underestimated the energy count.

Accurate counts on energy expended can help us lose weight. However, fitness device fans are still walking even though they know the counts might be off. The 10,000 daily steps we’re told to take for good health are supposed to equal five miles. An average person has a stride length of approximately 2.1 to 2.5 feet. That translates to just over 2,000 steps to walk one mile. 

The problem with that calculation is that it’s an approximation. Not everyone’s stride is the same. “The amount of steps it takes to do a mile depends on how large or small your stride is,” said Kelly Campoli, a fitness instructor who wears a Fitbit Flex. 

According to a report from the Physical Activity Council, out of 292 million Americans ages 6 years and older, 83 million live a sedentary lifestyle.

“Look, I know the study revealed that fitness trackers don’t give an accurate measure of how many calories are burned,” she said. “It doesn’t matter. What matters is that I keep moving. I lost weight and many people in my fitness class have too.”

“Most Americans who work in offices are lucky if they get up to 2,500 steps a day,” she said. “Having a 10,000 steps a day goal is worthwhile to me, the people in my class, and the many people who’ve purchased and wear their trackers daily.”

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