The calories you burn in any given period of time depend on what you are doing, your gender and your weight. Your physical condition also affects the rate at which your body burns calories, because your body needs more calories to maintain muscle than it does to maintain fat and other tissue. The American Council on Exercise offers a formula for determining how many calories you burn at different activities during a certain period of time.
Resting Metabolic Rate
Your resting metabolic rate is the amount of calories your body needs to simply sit still and do nothing. Your body requires these calories for your heart to beat and your body to function. According to the American Council on Exercise, the average person burns 0.02 calories per minute at rest, per kg of body weight. Men burn more calories at rest than women because their bodies generally contain more muscle mass.
Impact of Weight
To calculate your weight in kilograms, divide your weight in pounds by 2.2. Your weight in kilograms is approximately 73 if you weigh 160 lbs., 91 if you weigh 200 lbs. and approximately 57 if you weigh 125 lbs. This is the number you start with to determine how many calories you burn in 30 minutes.
Level of Activity
The next part of the equation measures the calories you burn at any given activity. For instance, for a weight of 160 lbs., or 73 kg, burning 0.02 calories per minute per kg at rest, you would multiply 0.02 by 73. You will burn 1.67 calories per minute sitting in a chair and doing nothing else at all. Comparatively, high-impact aerobics burn 0.12 calories per minute per kg, according to the American Council on Exercise. So if you weigh 160 lbs. and you participate in high-impact aerobics for one minute, you will burn 8.76 calories.
Total Calories Burned
To figure out how many calories you burn in a 30-minute period, multiply your weight in kg by the calories burned per minute, then by 30. Based on the premise that you weigh 160 lbs., this means that you burn approximately 50 calories in half an hour by breathing, or 1.67 times 30, and almost 263 calories for half an hour of high-impact aerobic exercise, or 8.76 times 30. The more strenuous the activity you engage in, the more calories you burn per kg per minute. If you engage in a moderate daily activity such as sweeping the floor, you will burn 0.05 calories per kg per minute, or 3.65 calories per minute if you weigh 160 lbs. and a little over 109 calories in half an hour, according to the American Council on Exercise.