Your body's energy comes in the form of calories. The more energy you need, the more calories you burn. Calculating how many calories you burn in a day must take into account your basal metabolic rate and your overall activity level. It must also factor in your exercise routines, which burn additional calories on the days that you work out.
Basal Metabolic Rate
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Your basal metabolic rate, or BMR, is the number of calories your body burns in one day to support basic body functions like respiration, circulation and immune system function. Calculation of BMR uses a person's weight, height and age. Your BMR accounts for only about 60 to 75 percent of the total daily calories you burn because it does not include the calories expended in activities and exercise.
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Harris-Benedict Equation
The Harris-Benedict equation adjusts your BMR to reflect activity and exercise levels by multiplying BMR by one of a set of activity multipliers. The multipliers reflect varying levels of activity throughout a one-week period. If you are sedentary and do not exercise, multiply your BMR by 1.2. If you exercise lightly one to three times per week, multiply by 1.375. If you exercise three to five days per week, multiply by 1.55. For exercise six or seven days per week, multiply by 1.725; if you exercise seven days a week and also have a physically demanding job, multiply by 1.9.
Use a Heart Rate Monitor
Heart rate monitors attach to your wrist and provide a real-time display of your heart rate. Because your heart rate can be used to calculate your rate of caloric expenditure, many monitors include calorie burn calculators. You can wear a heart rate monitor all day to determine how many calories you burn. A heart rate monitor takes into account any activities and exercises you engage in as well as your basal metabolic rate.
Higher or Lower Calorie Expenditure
The Harris-Benedict equation is still prone to error. Depending on your specific activity level and your metabolism, your caloric expenditure might be higher or lower than that obtained with the Harris-Benedict equation. If you work out several times a week in addition to your routine activities, you may burn additional calories. If you are trying to determine your calorie expenditure for weight loss or dietary reasons, talk to your doctor or nutritionist about your specific dietary needs before starting any diet or exercise plans.