Everyday Health Sensors: How Accurate are They?

If you cracked open a dictionary and looked up the word “couch potato” prior to me coming to Stanford, my name would have been written all over the page. So when I joined Stanford Women in Rugby and realized I could only run 0.2 seconds until fading from existence, I decided to start running everyday. I own an Apple Watch (thanks Costco sales!) and love tracking how many miles I have run and how many calories I burn from it. But I started to wonder just how much should I believe my watch…

Tracking your body’s health and activity is becoming easier and easier. With the integration of health monitors into popular gadgets like the Fitbit and the Apple Watch, people can access their heart rate, calories burned, steps taken, hours slept, and more with the flick of a finger. But how accurate are these gadgets’ measurements? Is there huge variability among the different gadgets available on the market?

How Do Health Trackers Work?

Many of these devices require the user to provide basic individual health information such as height, weight, gender, and age. Some devices then use these data to estimate your calories burned and stride length, which in turn can be converted to how much distance you cover.

In an experiment conducted by Iowa State University kinesiology professor Gregory Welk and then doctoral student Youngwon Kim, eight different fitness trackers were compared using a variety of statistics such as sleep, steps, calories burned, and distance ran. The researchers asked the participants to perform 13 different activities, ranging from running to playing basketball. Here is a breakdown of the results:


One of the more versatile devices for health monitoring is the Apple Watch, which provides a broader range of capabilities including messaging, audio playback, and other useful smartphone functions. It is also quite accurate. CNET writer Dan Graziano tested the Apple Watch’s step accuracy by walking on a treadmill for an hour, and then compared the mileage from the treadmill to the mileage tracked on the Apple Watch and on seven other activity trackers. He found that the Apple Watch had the smallest deviation, around 0.3%, while the Fitbit averaged around 9%.


After reading about the accuracy of these devices, I know that I should take those statistics with a grain of salt. Overall, if you want basic information about your day-to-day activity levels or health statistics, there is nothing wrong with using one of these devices to track that. However,  the studies detailed above have shown that there are significant differences in accuracy between products, and choosing one requires weighing your health-monitoring priorities and sacrificing some qualities for others. If you’re thinking about buying one of these to help you stay in shape, there is a lot to take into account.