The Metabolic Equivalent of Task (MET), or simply Metabolic Equivalent, is a physiological concept expressing the energy cost of physical activities [1] as multiplies of Resting Metabolic Rate (RMR) and is defined as the ratio of metabolic rate (and therefore the rate of energy consumption) during a specific physical activity to a reference rate of metabolic rate at rest, set by convention to 3.5 ml O2 . kg-1 . min-1 or equivalently 1 kcal (or 4.184 kJ) . kg-1 . hr-1. By convention 1 MET is considered as the resting metabolic rate obtained during quiet sitting. [2][3] MET values of physical activities range from 0.9 (sleeping) to 18 (running at 17.5 km/h).

Although the RMR of any specific person may deviate significantly from the above mentioned conventional reference value [4][5], MET values of physical activities provide a rough indication of the intensity of physical activities such as manual labour or exercise. MET is actually an index number and not an energy unit: a physical activity with a MET value of 2, such as walking at a slow pace (e.g., 3 km/hr) would require for a specific person twice the energy that person consumes at rest (e.g., sitting quietly).

More specifically, MET is used as a practical means of expressing the intensity and energy expenditure of physical activities in a way comparable among persons of different weight. Actual energy expenditure (e.g., in Kcal or Joules) during a physical activity depends on the person's body mass, therefore the energy cost of the same physical activity will be different for persons of different weight. However, since the RMR is also dependent on body mass in a similar way, it is assumed that the ratio of this energy cost to the RMR of each person will remain more or less stable for the specific physical activity and thus independent of each person's weight.

The 1-MET reference value of 1 kcal (or 4.184 kJ) . kg-1 . hr-1, is used by convention and refers to a typical metabolism at rest of an "average" individual. Even so, it must not be confused or misused as an approximation of Basal Metabolic Rate (BMR), which is the minimum metabolic rate obtained under specified conditions. This is illustrated by the fact that sleeping for instance has a MET of 0.9, while normal sleeping metabolism may be greater than the BMR.