The Resting Energy Expenditure or REE is the energy needed to maintain vital body processes such as breathing, food digestion, blood circulation, and sufficient energy for sedentary activities. Your REE increases with weight and height and decreases with age. Men usually have a higher REE than women (except during pregnancy). Stress (secretion of adrenaline), fever, drugs, (such as caffeine and amphetamines) increase REE.

So as to measure your daily energy requirements, a formula is used that factors in your so-called activity factor. This is essentially a number based on the level of physical activity selected from the alternatives below. The BMR is then multiplied by this activity factor, and the result is your daily energy requirements.