From my understanding, the BMR is just an educated guess as to how much calories you burn just to maintain yourself. The actual count could be more or less, but it would still be in that same general area.
The BMR formula has another formula to it that will take into account your current activity levels. If you are very sedentary, you will find that your BMR rate only slightly increases compared to someone very active.
Try searching google for BMR calculator. I forgot the website's name.