You can either do a search in Google and find a site that will calculate it for you, or you can follow the formulas below to work it out yourself:

English BMR Formula
Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) – ( 4.7 x age in years )
Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) – ( 6.8 x age in year )
 
Metric BMR Formula
Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) – ( 4.7 x age in years )
Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) – ( 6.8 x age in years )

Now, to take into consideration what level (if any) of exercise you do, to determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:

If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55 If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9

Ok, so that’s to work out how to maintain your current weight.  There are apparently 3500 calories to a pound, so over the course of a week you might need to reduce your calorie intake by around 500 calories a day if you want to lose weight.

There are of course “safe” and “unsafe” ways to do this, so please seek the advice of an expert first.  Starving yourself is NOT going to help you lose weight and keep it off, so it’s important your intake is appropriate for your body type and your goals!