Find out how many calories it takes to maintain your current weight by finding your basal metabolic rate or BMR. This is the number of calories that your body burns just to function. (It doesn't count calories you burn through exercise.)
This is the formula for women.
BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)
(Note: 1 inch = 2.54 cm and 1 kilogram = 2.2 lbs)
Take that result and multiply it by one of the numbers below depending on how active you are. That will give you the number of calories it takes for you to maintain your current weight. To reach your goal weight, you need to create a deficit of 500 calories a day. You can do this by cutting 250 calories a day and burning 250 calories more through exercise every day and you will lose a pound a week.
Sedentary = BMR X 1.2 (little or no exercise, desk job)
Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
Mod. active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
Extr. active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)