Everyone needs a different amount of Calories in their diet.  Unfortunately there is no perfectly accurate way to discover these needs.  Some Heart rate monitors will use a formula by monitoring your heart rate to give you an idea of how much you have burned, but do not count past a few thousand calories.  The next best thing is to calculate it yourself.  This system uses what we like to call close enough math, to give you the best guess of how many calories you need in a typical day.