There are a number of methods that can be used to calculate BMR ranging from the very scientific which is done in a doctor's office to an estimation that will work well for most people who are trying to get a handle on how much to eat.
Losing weight more quickly can be accomplished, but it is best done under a doctor's supervision, because quicker weight loss typically requires extreme caloric reduction that may fall under your BMR requirements.
Healthy diets to lose weight fast encompass a steady rate of weight loss of one to two pounds per week, meet your body's BMR requirements, and contain a variety of foods as recommended by the USDA food pyramid.
Eat sufficiently: Depriving your engines of food decreases BMR because it thinks there is an earthly famine lurking and thus conserves energy for ever-so-important factors like brain function.
You can calculate how your activity level affects your caloric needs by first calculating your basal metabolic rate (BMR), and then multiplying that amount by the Harris-Benedict equation.