According to a new study, those popular fitness trackers do a really good job of measuring your heart rate, but they aren't good at all when it comes to counting the calories you burn. Researchers at the Stanford Medical School who evaluated seven trackers reported their results yesterday.
The scientists say six of the seven devices had an error rate of less than 5% in heart rate measurements...but none measured energy expenditure accurately. The most accurate wrist monitor was off by an average of 27% and the least accurate was off by more than 90%.