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Study: Fitness Trackers Suck at Measuring Calories Burned

Researchers at Stanford University School of Medicine studied seven popular fitness trackers and discovered none of them could accurately measure energy expenditure.

By Angela Moscaritolo
Updated May 26, 2017
Stanford University School of Medicine fitness tracker study

Fitness trackers can give you an idea of how many calories you burned during a workout or entire day — but don't expect those measurements to be accurate.

Researchers at Stanford University School of Medicine studied seven popular fitness trackers and found that "none of [them] measured energy expenditure accurately," according to a news release. That finding applies to the Apple Watch ($300.00 at eBay) , Basis Peak, Fitbit Surge ($249.95 at Fitbit) , Microsoft Band, Mio Alpha 2 , PulseOn, and the Samsung Gear S2 ($99.60 at Amazon) .

The team found that even the most accurate of those devices was still off "by an average of 27 percent." The least accurate was off by an impressively bad 93 percent.

"People are basing life decisions on the data provided by these devices," the study's senior author Euan Ashley, a professor of cardiovascular medicine, genetics, and biomedical data science at Stanford, said in a statement. But doing so might not be a wise decision.

Ashley and the team enlisted sixty volunteers — 31 women, 29 men — who wore the seven devices while walking or running on treadmills or using stationary bikes. They measured each volunteer's heart rate with a medical-grade electrocardiograph and measured their metabolic rate with an instrument used for measuring the oxygen and carbon dioxide in breath. The researchers then compared the results from the wearable devices with the measurements from the two "gold standard" instruments.

The team found that six out of the seven devices measured heart rate "within 5 percent." It was a very different story for their measurements of calories burned.

"The heart rate measurements performed far better than we expected," Ashley said, "but the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me."

So according to the release, "basing the number of doughnuts you eat on how many calories your device says you burned is a really bad idea."

A paper reporting the researchers' findings was published Wednesday in the Journal of Personalized Medicine.

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About Angela Moscaritolo

Managing Editor, Consumer Electronics

I'm PCMag's managing editor for consumer electronics, overseeing an experienced team of analysts covering smart home, home entertainment, wearables, fitness and health tech, and various other product categories. I have been with PCMag for more than 10 years, and in that time have written more than 6,000 articles and reviews for the site. I previously served as an analyst focused on smart home and wearable devices, and before that I was a reporter covering consumer tech news. I'm also a yoga instructor, and have been actively teaching group and private classes for nearly a decade. 

Prior to joining PCMag, I was a reporter for SC Magazine, focusing on hackers and computer security. I earned a BS in journalism from West Virginia University, and started my career writing for newspapers in New Jersey, Pennsylvania, and West Virginia.

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