The number of calories you need to eat to maintain your weight is equal to the number of calories you burn. Your activity level increases your calorie expenditure rate, as does your body weight. Your calorie expenditure rate decreases as you age, and men generally have a higher calorie expenditure rate than women.
Basal Metabolic Rate
Your basal metabolic rate is your calorie expenditure rate when you're at complete rest. This means that you're not digesting food and your external environment is at room temperature, so that you don't have to use calories to heat or cool your body.
Activity Factor
An activity factor is a number that represents your activity level. Your activity factor while you're at complete rest is one, and most lifestyles have an activity factor between one and two. For example, a person with a generally sedentary lifestyle has an activity factor of 1.2 and a person who engages in heavy exercise every day has an activity factor of 1.725.
Formula
Weill Cornell Medical College provides the Harris-Benedict equations to calculate your BMR. The Harris-Benedict equation for women is BMR = (1.850 x Height) + (9.563 x Weight) - (4.676 x Age) + 655.1. The corresponding equation for men is BMR = (5.003 x Height) + (13.75 x Weight) - (6.775 x Age) + 66.5. Your height must be in centimeters and your weight must be in kilograms.
Limitations
Muscle requires more energy to maintain than fat. The Harris-Benedict equations assume that you have an average body fat percentage. They'll tend to produce a BMR that's too low if you have a low body fat percentage. Similarly, the Harris-Benedict equations will provide a BMR that's too high if you have a high body fat percentage.
Calculations
This example assumes you're a 33-year-old male. You're 179 cm tall and weigh 74 kg. You therefore have a BMR of (5.003 x 179) + (13.75 x 74) - (6.775 x 33) + 66.5 = 1756 calories per day. Multiply your BMR by your activity factor to get your daily calorie expenditure rate. Assume you have an activity factor of 1.725 for this example. Your daily calorie expenditure is therefore 1756 x 1.725 = 3029 calories per day.
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