12 X your bodyweight in lbs is a good rule of thumb, but if you really want to calculate yours, there's only one way for sure. Everyone's a little different. Take a week. Weigh first thing Sunday morning, and then write down every single thing you eat. You have to get an exact calorie count. Don't miss anything. Do this for an entire week, and make sure you get everything. then Weigh at the same time the next sunday morning. If you weigh exactly the same, then that's your maint calories. If you've gained, take what you gained and multiply by 3500 and subtract. If you've lost, add the number. Do that for a few weeks, and you'll have a good idea of your BMR calories.