Use the calculator to determine an approximate value for the number of calories per day that your body needs to maintain the weight specified. It takes about 3,500 calories over what your body needs to gain one pound. Conversely, for every 3,500 calories burned through exercise or removed from your plate, your body will loose one pound. For every pound of muscle you add to your body through exercise, you body will burn an extra 30 to 40 calories per day.