Face your roof so the roof looks like a triangle.
Measure from one end of the house to the other end of the house, then divide by two to calculate your run. For example, imagine the house measures 20 feet, so the run is 10 feet.
Measure from the steepest point of the roof downward to where the roof starts. This is your rise. In the example, assume the rise is five feet.
Divide the rise by the run. In the example, five feet divided by 10 feet equals a pitch of 0.5.