Embodiments of the present disclosure take advantage of a modified Predicted Heat Strain Model (PHSM) and heat balance equation to accurately determine a calorie expenditure. In various embodiments this is accomplished by using a novel sensor system to gather sensor data comprising a sweat rate and ambient conditions and applying the sensor data to a modified PHSM that distinguishes between normal cases and extreme cases in which body heat can and cannot be efficiently dissipated through sweating. By using a dual method to calculate metabolic rate based on these conditions, the PHSM allows to accurately determine a calorie expenditure.