There is a rule of thumb out there that states 3500 calories equals 1 pound. This has been generally accepted by most. With that equation you can decrease you calorie intake over a week and predict weight loss. Recently there has been some questions about that formula. Check out the following post for a good article with more details about this very subject.

http://www.atozfitness.com/wordpress/3500-calories-to-lose-a-pound-is-this-formula-all-wrong/