A unit, based on temperature difference and time, used in estimating cooling energy consumption and specifying nominal cooling load of a building in summer. For any one day, when the mean temperature is more than 65 degrees F (18 degrees C), there are as many degree days as there are degrees Fahrenheit (Celsius) difference in temperature between the mean temperature for the day and 65 degrees F (18 degrees C). Annual cooling degree days (CDD) are the sum of the degree days over a calendar year.