You must have often heard people trying to lose weight say that they have reduced their food intake and exercise more. However, the exact amount of reduction in calorific intake and increase in physical activity that should be done, depends on a lot of other parameters to get you the best result in the quickest possible time frame. If ones approach to this is not planned with utmost care, the results can often be very disappointing.
A simple calculation in terms of calorific intake for weight loss or gain would be:
Calorific requirement of your body(per day) > Calorific Intake(per day) = Eventual weight loss.
Calorific requirement of your body(per day) < Calorific Intake(per day) = Eventual weight gain.
This is obviously an oversimplified statement. Since every person and every body is different, its advisable to consult your personal trainer and/or dietitian for the best course of action.