The cooling degree day is unit used to relate the day's temperature to the energy demands of air conditioning. Cooling degree days are calculated by subtracting 65°F (18°C) from a day's average temperature. For example, if the day's high is 90°F and the day's low is 70°F, the day's average is 80°F. Eighty minus 65 is 15 cooling degree days.

Cooling degree days can be used to compare the current summer to past summers. Hence, by studying degree day patterns in your area, you can evaluate the increases or decreases in your heating or air-conditioning bills from year to year.