Fitness freaks, listen up: Your activity tracker is not as accurate as you think it is.

Researchers at Iowa State University tested eight different activity monitors to see just how accurate each model really is. They had 30 men and 30 women wear all the devices — as well as a portable metabolic analyzer as the comparison — during a 69-minute activity session, which included everything from writing at a computer to running to playing Wii tennis.

The result? Most trackers were reasonably accurate, their calories-burned estimates landing within 10 to 15 percent of the actual number. But the study did reveal that some models are more accurate than others. Choosing a tracker shouldn’t be about which one looks the coolest; function is what matters.

Read more on OZY.com: Don't Trust Your Fitbit | Acumen | OZY