CBS Local — If you got a new fitness tracking gadget during the holidays, a new study has some bad news for just how useful it’ll be after your next workout. According to Stanford University, fitness monitoring devices like the Apple Watch and Fitbit proved to be largely inaccurate at counting how many calories you burn off.

Researchers say that while the devices were very accurate at measuring a person’s heart rate while working out, the models Stanford tested were off by as much as a shocking 93 percent when measuring calories burned.

“The heart rate measurements performed far better than we expected,” said lead author Euan Ashley in a university release. “But the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me.”

Stanford tested the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2. After tracking 60 volunteers, who were working out on treadmills and stationary bicycles, the study found that none of the devices gave the correct number of calories burned when the results were compared to a medical-grade electrocardiograph.

The results, published in the Journal of Personalized Medicine, report that the fitness trackers were off by an average of 27 percent. The researchers add that people are likely making many bad decisions regarding their diet and exercise routine because of the false readings. “People are basing life decisions on the data provided by these devices,” Ashley added.