The number of calories that you need to maintain your weight depends on physical factors and your activity level. You can estimate your calorie requirements from standard formulas that make varying assumptions about your physiology. You'll typically perform these calculations when you design a weight loss plan.
Basal Metabolic Rate
Your basal metabolic rate is the minimal rate at which you expend calories. This is your calorie expenditure rate when you aren't performing any activity. The basal metabolic rate assumes you're in a deep sleep during the post-absorptive phase of digestion, meaning that you're not actively digesting food.
Harris-Benedict Equation
The Harris-Benedict equations are a pair of equations that estimate the basal metabolic rate for both men and women. The equation for women is (1.850 x height) + (9.563 x weight) - (4.676 x age) + 655.1 where height is in centimeters, weight is in kilograms and age is in years. The equation for men is (5.003 x height) + (13.75 x weight) - (6.775 x age) + 66.5. Both equations provide the basal metabolic rate in units of calories per day.
Activity
You must multiply your basal metabolic rate by your activity factor in order to determine your actual calorie expenditure. The activity factor is 1.2 sedentary people and 1.375 for people who engage in light exercise as often as three times per week. The activity factor for moderately active people is 1.55, 1.725 for people who engage in heavy exercise at least six times per week and 1.9 for people with jobs requiring hard physical labor.
Body Fat
The Harris-Benedict Equation assumes you have an average body-fat percentage. It's therefore less accurate for people who are unusually muscular or fat. Muscle burns more calories than fat, so the Harris-Benedict Equation will provide a low value for people with a low body-fat percentage and a high value for those with a high body-fat percentage.
Katch-McArdle Formula
The Katch-McArdle formula is a formula that provides your basal metabolic rate from your body weight that's not due to fat, also known as your lean mass. This formula is (21.6 x lean mass) + 370 where your lean mass is in kilograms.
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