Sometimes I just tell people to multiply their current bodyweight by 12 (as a general rule of thumb), but if you want to be more precise:
Harris-Benedict Equation for BMR
Men: 66 + (13.7 x W) + (5 x H) - (6.8 x A)
Women: 655 + (9.6 x W) + (1.7 x H) - (4.7 x A)
W: Weight in kg (lbs./2.2)
H: Height in cm (in. x 2.54)
A: Age in years
Notice you have to convert to the metric system to plug in your W and H values. This is your BMR value only and does not account for any caloric expenditure in response to exercise. Also, remember a pound of bodyfat carries 3,500 calories so from your BMR, you can calculate for 1-2 lbs. of bodyfat per week.
Hope this helps...