The number of calories a person needs per day varies based on height, weight, gender, age and activity level. To calculate the total number of calories a person needs per day, take into account the body's basal metabolic rate, the person's physical activity level and the thermic effect of food.
The basal metabolic rate is the amount of energy a body needs to function at rest, which accounts for about 60 to 70 percent of the calories burned in a day. Physical activity burns calories, but the number of calories burned in a given activity depends on the person's weight. The thermic effect of food is the amount of energy the body uses to digest the food eaten.