Your basal metabolic rate (BMR) is the number of calories your body burns in a day when operating at absolute minimum capacity. Basically, imagine sitting on the couch all day. Your BMR then, is the number of calories your body burns just to maintain your heartbeat, digestion, respiration, tissue repair and other organ functions. Thats it.
BMR is influenced by a number of factors, including age, weight, height, gender, environmental temperature, and diet and exercise habits. Because of these varying factors, its hard to pin down BMR to the precise calorie, but we can get pretty close. Here are some formulas for figuring out your BMR. Get out your calculator and plug your numbers into whichever formula applies to you.
MALE: 66 + (6.3 x body weight in pounds) + (12.9 x height in inches) - (6.8 x age in years)
FEMALE: 655 + (4.3 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
Your BMR accounts for only about 75% of your total daily caloric expenditure; you can also increase your BMR with exercise.