The Apple Watch and other smartwatches are not reliable for calorie tracking, despite accurate heart rate measurements. A 2024 Stanford University study of 60 volunteers testing seven wearables including Apple Watch, Fitbit Surge, Microsoft Band, and Samsung Gear S2 found energy expenditure measurements were off by 27% to 93% compared to medical-grade equipment. Dr. Euan Ashley, senior author, said the magnitude of inaccuracy was surprising.

Wearables cannot directly measure metabolism; they estimate calories using proxy data like heart rate, motion sensors, GPS, and machine learning algorithms. Anna Shcherbina, assistant professor on the Stanford study, notes algorithms struggle to be accurate across diverse individuals due to variables like fitness level, height, and weight. Providing personal data helps but cannot overcome inherent limitations.

A recent Nature study confirmed these findings, noting multiple sensor inputs compound errors, and factors like skin tone and moisture affect readings. Smartwatches detect exercise types via accelerometer, gyroscope, GPS, and heart rate, using machine learning to identify activities like walking, rowing, or cycling. However, calorie estimates remain educated guesses rather than precise measurements.