BMR stands for Basal Metabolic Rate, and is essentially, the minimum calorie requirement needed to maintain the life of a sedentary animal. If you were asleep for 24 hours a day, the BMR is the amount of energy that your body would burn. There are certain factors which affect how the BMR functions in each person.
Age: The BMR is higher in younger people. Clearly, with age, the metabolic rate slows, and so older people have a different BMR ratio than younger people with identical weight and height.
Height. Taller people have, in generally, higher BMR's than shorter.
Body Composition: Although two people may weigh the same, they can have very different appearances. One person may have a lot of lean muscle tissue, another may have a lot of fat. Generally, as BMR calculations do not take into account what your composition is, this factor is not important when working out a BMR for calorie counting.
It is also important to remember that fasting and starving can significantly reduce the BMR, so if you are calculating this in order to diet, be careful not too cut back too much.
There are two major methods of working out your BMR. The first is known as the 'General Calculation'. In this, the BMR is equal to your body weight in pounds, times by 10. For example, 200 lbs times 10, equals 2000. This is fairly simple, and combined with a simple calculation for activity levels, should enable you to lose weight through calorie counting. However, this general calculation is an approximation method. In order to work out exactly how much you need to maintain your weight, and how much you need to loose, you need to try out different methods.
A more sophisticated way of calculating BMR is known as the Harris Benedict Equation. This involves two separate methods of calculation, depending upon the sex of the person involved. For a male the equation would be 66 + (13.7 x weight) + (5 x height) ' (6.8 x age). So a 200 pound 27 year old man who is 5.10 will have a BMR of 4300. This is the amount he has to eat in order to maintain his current weight. If he wanted to lose weight, at an estimate of one pound a week just by diet, he would eat 3800.
A woman's BMR score is revealed through the equation: 655 + (9.6 x weight) + (1.7 x height) ' (4.7 x age). So a 200 pound 27 year old woman who is 5.10 will have a BMR of 2567.1. This is the amount that she would have to eat in order to maintain her weight. If she wanted to lose weight, again purely through diet, then she would reduce her intake by 500 (3,500 calories equals one pound) calories per day. This would make her intake 2067.1 per day.
You can see the dramatic differences between the two weights, and also the difference between the General calculation, and the Harris-Benedict equation on the other. In practice, the Equation is the best device to work out a more accurate BMR for each person, and also allows the person to recalculate as their weight changes, so you are not always calculating the same amount.
If you feel that the Harris Benedict Equation is not accurate enough for you, then you can also choose to measure your BMR in other ways, including the Underwater Weighing measurement, which is the gold standard for calculating body composition, and you can also do a Skinfold Measurement test, which measures fat folds around the waist, back and other areas. These are the most extreme versions of the BMR calculation spectrum.