The amount of calories your body needs at rest, called the basal metabolic rate, or BMR, varies depending on age, gender, weight, height, muscle mass, body fat, diet, health, external temperature, exercise habit and genetic predisposition. There are several different ways to calculate your BMR, of varying accuracy.
Total Daily Energy Expenditure
For diet planning, the number you want to know is your total daily energy expenditure, or TDEE. Your TDEE is the overall number of calories your burn in a day, both when you are active and when you are at rest. The average in the United States for women is 2,000 to 2,100 calories per day, and for men it is 2,700 to 2,900 per day. The TDEE tells you your maintenance level, that is, roughly how many calories you need to consume to maintain your current weight. If you eat less, you will lose weight, and if you eat more, you will gain weight. The problem with using a very general average is that it will not be accurate for people who are extremely active or very inactive, and it doesn't allow for individual variation.
The Harris-Benedict Formula
The Harris-Benedict formula calculates your BMR based on your height, weight and age. For men, BMR = 66 + (13.7 x weight in kilograms) + (5 x height in centimeters) - (6.8 x age in years). So if you are a 190 cm tall 40-year-old man weighing 70 kg, you would calculate your BMR as 66 + (13.7 x 70) + (5 x 190) - (6.8 x 40) = 66 + 959 + 950 - 272 = 1,703. That means that a man of this age, height and weight who did nothing but lie in bed all day would need 1,703 calories a day to maintain his weight. The formula for women is slightly different: BMR = 655 + (9.6 x weight in kilograms) + (1.8 x height in centimeters) - (4.7 X age in years).
Katch-McArdle Formula
The Katch-McArdle Formula is a more accurate way of calculating BMR than the Harris-Benedict formula because it takes into account that lean body mass is more metabolically active than is fat. A 200 lb. bodybuilder with 7 percent body fat burns more calories simply lying down asleep than a 200 lb. sedentary individual with 35 percent body fat. The Katch-McArdle formula is BMR = 370 + (9.79759519 x lean mass in pounds). Calculate your lean body mass according to this formula: lean body mass = weight - (weight x percent body fat/100). If you weigh 200 lb. and have 15 percent body fat, your lean body mass would be 200 - (200 x .15) = 200 - 30 = 170.
Increasing BMR
You can't do anything about your genetic heritage, age or gender, but you can increase your BMR. Because your BMR increases with lean body mass, resistance training, which increases your muscle mass, increases BMR. Exercise also elevates your BMR. Even minor increases in activity -- short walks after meals, parking farther from your office or taking stairs instead of elevators -- not only burns calories but increases BMR. Finally, starvation diets will actually lower your BMR, making them counterproductive for weight loss. Instead, if you are concerned about your weight, consult a health care provider or registered dietitian to plan a healthy lifestyle and diet for your individual needs.
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