If I want to lose one pound of body fat I have to burn 3500 calories. Normally that would mean a calorie reduction of about 500 calories a day from what my current intake would be. A 20% reduction of CPD (Clalories Per Day) is a healthy reduction and also is small enough to give you the neccessary calories your body needs . Let's look at the numbers:
If you normally take in 2500 cals a day a 20% deficit would equal 500.
2500 X 20% = 500. 500 cpd X 7 days = 3500. 1 lbs a week
With exercise you should be able to burn or reduce about 7000 calories per week. It's important to note losing more than 1-2 lbs per week is not very healthy. When you're losing more then 1-2 lbs/wk your are in danger of losing muscles and you absolutely do not want to lose any muscle.