BMR stands for Basal Metabolic Rate, and is essentially, the minimum calorie requirement needed to maintain the life of a sedentary animal. If you were asleep for 24 hours a day, the BMR is the amount of energy that your body would burn. There are certain factors which affect how the BMR functions in each person.
Age: The BMR is higher in younger people. Clearly, with age, the metabolic rate slows, and so older people have a different BMR ratio than younger people with identical weight and height.
Height. Taller people have, in generally, higher BMRs than shorter.
Body Composition: Although two people may weigh the same, they can have very different appearances. One person may have a lot of lean muscle tissue, another may have a lot of fat. Generally, as BMR calculations do not take into account what your composition is, this factor is not important when working out a BMR for calorie counting.
It is also important to remember that fasting and starving can significantly reduce the BMR, so if you are calculating this in order to diet, be careful not too cut back too much.
There are two major methods of working out your BMR. The first is known as...