When it comes to tracking calories, wearables like Apple Watch and Fitbit often provide very inaccurate information, according to Stanford University researchers.

The study, published in the Journal of Personalized Medicine, evaluated seven devices (Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2) using 60 volunteers on treadmills and stationary cycles. Rather than comparing the devices to each other, researchers compared their accuracy to the U.S. Food and Drug Administration's standards for medical devices.

What they found is some devices, particularly Apple Watch, are getting more accurate at tracking a user’s heart rate. For measuring calories, however, the data provided was way off—the most accurate device had an average error rate of 27 percent.

“For a lay user, in a non-medical setting, we want to keep that error under 10 percent,” said study author and Stanford graduate student Anna Shcherbina, PhD.

Her best guess as to why energy expenditure measurements were so incorrect would be the variables affecting each device’s algorithm, as each user’s fitness level, height and weight could affect those results.

The take-home message for both users and practicing physicians, according to co-author Euan Ashley, DPhil, is the heart rate measurements from consumer-level wearables are largely reliable, which goes against results from an earlier study released by the Cleveland Clinic.

“The heart rate measurements performed far better than we expected,” Ashley said, “but the energy expenditure measures were way off the mark.”