New research says certain monitors may over- or under-estimate your burned calories.

By Liz Steelman
Updated August 20, 2015
Matthew Leete

If you’ve found your Fitbit badge collection isn’t correlating with your waistline, it might be because your fitness tracker isn’t working as hard as you think. According to a new study from Iowa State University, fitness monitors are, on average, off by 15.3 to 30.4 percent.

For the study, published in Medicine & Science in Sports & Exercise, the journal of the American College of Sports Medicine, researchers equipped 52 people ages 18 to 65 with eight total fitness monitors from consumer to medical grade. They then spent the next 80 minutes performing flights of sedentary, aerobic and resistance activity with rest periods in between. The energy expenditure from the active time was measured by all seven monitors and tested against Oxycon Mobile, a portable metabolic system.

Overall, the research monitor trackers had the lowest error rates—the BodyMedia Core had an error rate of 15.3 percent and Actigraph GT3X+ had one of 16.7 percent. Of the consumer models, Fitbit Flex was close behind with 16.8 percent and Nike+ FuelBand SE and Jawbone UP24 with 17.1 and 18.2 percent, respectively. The monitor with the highest overall error rate was Mistfit Shine with 30.4 percent. It also had an especially high error rate—60.1 percent—when measuring aerobic activity.

All of the monitors tested were especially off when it comes to measuring resistance activity, with an error rate that ranged from 20 (Nike+ FuelBand SE) to 52.6 percent (Jawbone UP24).

Though accuracy is a factor in improving fitness, the researchers say it's not the only benefit of having a fitness monitor.

"I think the key to a consumer is not so much if the activity monitor is accurate in terms of calories, but whether it's motivational for them and keeps them accountable for activity in a day," Greg Welk, researcher and professor of kinesiology, said in a statement.