This calorie calculator uses Metabolic Equivalent of Task or MET values, which are essentially the energy cost of doing a physical activity. MET values are multiplies of Resting Metabolic Rate or RMR and are 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 when you are at rest.

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.

Keep in mind that MET values are statistically pulled from a sample group of individuals and are extrapolated to be indicative averages. There are a lot of factors that will dictate the exact metabolic rate of an activity for each individual such as fitness levels, adiposity, cardiovascular health, or even ambient temperature.

More information about METs values -