To lose 1 pound of body fat, one should create a calorie deficit 3,500 Cal. For example, if your goal is to lose 2 pounds a week, you should create a calorie deficit 7,000 Cal. a week or 1,000 Cal a day.
Burning off 300 extra Cal. through exercise and taking in 700 Cal. less than your body needs, theoretically, you will burn the desired 2 pounds a week.
Practically, it does not always work this way. For example, your calorie intake is mostly carbohydrate calories, and your exercise is only aerobic exercise (walking, running, swimming, etc.) - than your appetite will be increased proportionally the energy you spend exercising and it'll be very hard on you to stick to the 700-Cal. deficit.
If, however, you do weight lifting and limit your carbohydrate intake, the calorie deficit you need for your goal can be not 700 but only 400 Cal. and your appetite will be decreased so you can eat all you want - but you want less and the 300-Cal. intake limitation comes naturally!
