The metabolic rate, as a conversion of chemical into mechanical and thermal energy, measures the energetic cost of muscular load and gives a quantitative estimate of the activity. Metabolic rate is an important determinant of the comfort or the strain resulting from exposure to a thermal environment. In particular, in hot climates, the high levels of metabolic heat production associated with muscular work aggravate heat stress, as large amounts of heat need to be dissipated, mostly by sweat evaporation. On the contrary, in cold environments, high levels of metabolic heat production help to compensate for excessive heat losses through the skin and therefore reduce the cold strain. This International Standard specifies different methods for the evaluation of metabolic rate in the context of ergonomics of the thermal working environment. It can also be used for other applications for example, the assessment of working practices, energetic cost of specific jobs or sport activities, the total energy cost of an activity, etc. The estimations, tables and other data included in this International Standard concern the general working population. Users should make appropriate corrections when they are dealing with special populations including children, aged persons, people with physical disabilities, etc. Personal characteristics, e.g. body mass, may be used if the body is moved due to walking or climbing (Annex A and B). Gender, age and body mass are considered in Annex C for the evaluation of the metabolic rate from heart rate.