The Harris-Benedict equation, also known as the Harris-Benedict principle, is used to estimate what a person's BMR (basal metabolic rate) and daily requirements are.
Your BMR total is multiplied by another number which represents your level of physical activity. The resulting number is your recommended daily calorie intake in order to keep your body weight where it is.
This equation has some limitations. It does not take into account varying levels of muscle mass to fat mass ratios - a very muscular person needs more calories, even when resting.
BMR Equation
Male adults
66.5 + (13.75 x kg body weight) + (5.003 x height in cm) - (6.755 x age) = BMR
66 + ( 6.23 x pounds body weight) + ( 12.7 x height in inches ) - ( 6.76 x age) = BMR
Female adults
55.1 + (9.563 x kg body weight) + (1.850 x height in cm) - (4.676 x age) = BMR
655 + (4.35 x kg body weight) + (4.7 x height in inches) - (4.7 x age) = BMR