Determination of energy needs is a fundamental part of nutrition support. The amount of metabolically active tissue mass is the major determinant of metabolic rate. The other components of total energy requirement in an adult are physical activity, diet-induced thermogenesis, and illness hypermetabolism. Measurement with indirect calorimetry is possible but not common. Measurement can capture the effect of body size, diet-induced thermogenesis, and illness on metabolic rate but usually not the effect of physical activity. More often, the energy need is calculated based on its association with body weight and composition. Many equations have been proposed over the years, as have adjustments to body weight in an attempt to capture the distorting effect of body composition in obesity and emaciation. Some equations capture the effects of illness and diet-induced thermogenesis without the need for modification; some require multiplication with various factors. None predict the energy expenditure from physical activity. In determining the energy prescription, all of the component parts must be considered, regardless of whether energy expenditure is measured or calculated.