In other words, your BMR is all the energy used for the basic processes of life itself. BMR usually accounts for about two-thirds of total daily energy expenditure. BMR may vary dramatically from person to person depending on genetic factors. It is very important to note that the higher your lean body mass is, the higher your BMR will be. This is very significant if you want to lose body fat because it means that the more muscle you have, the more calories you will burn.
Muscle is metabolically active tissue, and it requires a great deal of energy just to sustain it.
In this manner it could be said that weight training helps you lose body fat, albeit indirectly. This makes it more accurate than determining calorie needs based on total bodyweight alone. The only variable it does not take into consideration is lean body mass. You weigh 120 lbs. If you have had your body composition tested and you know your lean body mass, then you can get the most accurate BMR estimate of all.