Basal Metabolic Rate ( BMR ) is the number of calories you'd burn if you stayed in bed all day in neutrally temperate environment. The release of energy in this state is sufficient only for the functioning of the vital organs, such as the heart, lungs, brain and the rest of the nervous system, liver, kidneys, sex organs, muscles and skin.
The BMR formula uses the variables of height, weight, age and gender to calculate the BMR. This is more accurate than calculating calorie needs based on body weight alone. The only factor it omits is lean body mass and thus the ratio of muscle-to-fat a body has. Remember, leaner bodies need more calories than less leaner ones.

This is the BMR formula :

For women:
655 + (4.3 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years) = BMR


For men:
66 + (6.3 x weight in pounds) + (12.9 x height in inches) - (6.8 x age in years) = BMR