Be careful about relying on fitness trackers to count calories. A study of several top brands—Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2—found them to be off by 27% to 93%. The 2017 research, from the Standford University School of Medicine, was published in the Journal of Personalized Medicine. In 2018, UK researchers reviewed 60 studies on the topic and found some brands better than others, but as a whole they didn’t count calories very accurately. “Our paper shows that estimates are often poor and they vary depending on the activity being performed,” said lead study author Ruairi O’Driscoll of the University of Leeds in the U.K.