Your body burns calories every minute of every day, whether you're asleep, watching television, working at the computer or working out. While the rate that you burn calories varies by your age, weight, sex and activity level, your body requires a certain number of calories daily to maintain your bodily functions -- while just breathing. This is your basal metabolic rate.
Calculating Your BMR
A relatively accurate method of calculating your BMR is the Harris-Benedict formula. This complicated formula requires calculating your height and weight into centimeters and kilograms, then factoring in your age according to the formula. This gives you your basic BMR.
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Calories Burned While Breathing -- or Sleeping
Instead of using a specific formula, you can simply look up the average number of calories burned on a chart, such as the one in the Harvard Heart Letter. According to the chart, in 30 minutes of sleep a 125-pound person burns 19 calories, while a 185-pound person burns 28 calories. Sleeping is the one time that you can be sure that you're only breathing.