Your basal metabolic rate, or BMR, is the number of calories you burn every day while at rest. These calories are required simply to perform basic functions and don't include the calories you need for exercise or activities. Knowing your BMR can be useful in determining how many calories you need to burn to lose weight.
Using the Formula
Use the proper formula to determine your BMR. For women, this formula is (4.35 x body weight in pounds) + (4.7 x height in inches) - (4.68 x age) + 655. For men, the formula is (6.25 x body weight in pounds) + (12.7 x height in inches) - (6.76 x age) + 66. The number that you come up with is the number of calories you would burn per day were you to remain sedentary and do nothing more than sit or lie down.
Factoring in Activity
While BMR is a good starting point, it isn't that useful unless you stay in bed all day because it doesn't account for your activity levels. To work out how many calories you do burn per day, use the Harris-Benedict equation. This involves multiplying your BMR by an activity factor. If you're sedentary, multiply it by 1.2, and if you're a little more active, multiply it by 1.375. Those who are moderately active or very active need to multiply by 1.55 and 1.725, respectively, while extra active individuals need to multiply by 1.9. This gives you a good idea of how many calories you burn in an average day, which is known as your maintenance level.
Calories for Weight Loss
To lose weight, you have to consume fewer calories than you burn. If you're in the normal weight range and only want to lose 5 to 10 pounds, you need to take 500 to 700 calories from your maintenance level, advises Manuel Villacorta of the Academy of Nutrition and Dietetics. For overweight individuals, this rises to 700 to 1,000 calories daily, while obese persons can go for a 1,000 to 1,500 per day deficit. You don't want to try for a bigger deficit than this, however, as going too low with your calorie intake can lead to lethargy, reduced immune function, an unhealthy relationship with food and large gains in weight post-diet.
Use BMR as a Base
As useful as BMR is, it is still only a theoretical number -- every person works differently and the actual number of calories you burn may be higher or lower than the formula gives. Additionally, to lose weight, knowing your BMR is not enough -- you need the activity factor, too. As you lose weight, your BMR will decrease as your body has to do less work to keep you going. Therefore, you may need to recalculate your BMR and reduce your calories further to keep losing weight.