Determine the location for which you want to calculate the full day equivalents. This can be as large an area as a state or as particular as the location of a certain weather station. It is up to you as to which type of information suits your purpose.
Acquire the needed heating and cooling degree day information in the time frame in which you are interested. If it is annual data, note that the heating degree year runs from July 1 to July 1, while the cooling degree year is the calendar year. Note that the degree day calculations may be the result of a simple averaging of the high and low temperatures for each day or through the use of a sophisticated integrating software program. The more accurate calculations also can be obtained for baselines other than 65 degrees Fahrenheit or in Celsius temperatures.
Obtain full-day equivalent temperatures by dividing the heating or cooling degree days in the period for which you are calculating by the number of days in that period. For example, if there are 70 degree days in a week, that means there are 10 degree days for each day. This means each day was equivalent to a 55-degree average temperature day (heating degree days) or a 75-degree average temperature day (cooling degree days). Note that while this process works for any period, it is more meaningful for shorter periods, since the zero degree days skew the computation.