The number of calories you burn in the day depends on your activity level and personal factors such as age, weight and height. Each gender has its own formula to calculate daily calorie expenditure. This is a common calculation to perform when you wish to predict your weight loss rate while following a fitness program.
Basal Metabolic Rate
Your basal metabolic rate, or BMR, is the rate at which you expend calories while you're at complete rest. It assumes you're at room temperature and not actively digesting food, which normally means 12 hours of fasting. Your BMR is essentially the minimum calorie expenditure rate that you need to live.
Activity Level
You generally burn calories at a much higher rate than your BMR. It's therefore necessary to determine your activity level in order to calculate your calorie expenditure rate. For example, an activity level of 1.2 indicates a generally sedentary lifestyle. An activity level of 1.375 applies to people who engage in light exercise up to three times a week. An activity level of 1.55 indicates that you're moderately active. People who exercise vigorously up to six days a week have an activity level of 1.725 and those with a physically demanding job have an activity level of 1.9.
Formula
The Harris-Benedict equations calculate your BMR in units of calories per day.
The equation for women is BMR = 655.1 + (9.563 x weight) + (1.850 x height) - (4.676 x age), where weight is in kilograms, height is in centimeters and age is in years.
The corresponding equation for men is BMR = 66.5 + (13.75 x weight) + (5.003 x height) - (6.775 x age).
Multiply your BMR by your activity level to get your daily calorie expenditure rate.
Calculation
Assume you're a 38-year-old woman weighing 73kg with a height of 172cm. Your BMR is 655.1 + (9.563 x 73) + (1.850 x 172) - (4.676 x 38) = 1,494 calories per day. You're moderately active, which gives you an activity level of 1.55. Your daily calorie expenditure is 1,494 x 1.55 = 2,315 calories per day.
Weight Loss
The National Institutes of Health recommend that you limit your weight loss to 2 lbs. per week. A pound of fat has 3,500 calories, so 2 lbs. of fat is equal to about 7,000 calories. You should therefore burn no more than 1,000 calories per day more than you consume.
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