Calculate your basal metabolic rate -- the amount of calories you burn from basic biological functions -- using the appropriate formula. The formula for women is 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years ). For men the formula is 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year ).
Multiply your BMR by 1.2 to get your daily caloric needs based on a sedentary lifestyle. This will give you the number of calories that you need to consume to maintain your current weight.
Consume 500 to 1,000 fewer calories per day than your daily needs for maintenance. This will add up to weight loss of 1 to 2 pounds per week, meaning you can expect to lose 50 pounds in 25 to 50 weeks.