The heating degree day measure was created by heating engineers. Heating degree day (HDD) and cooling degree day (CDD) are quantitative indices demonstrated to reflect demand for energy to heat or cool buildings. The bases to terminate whether a specific day is a heating degree day or a cooling degree day is the daily average temperature TM (add the high and low temperatures and divide it by two). Each day with a mean temperature below 18°C is counted as one heating degree day. The cooling degree day is the summer counterpart of the heating degree day.

Heating degree days are calculated over a period of time (typically a year) by adding up the differences between each day's mean daily temperature and the temperature of 18°C (or 65°F). For any temperature above 18°C the building is assumed not to need any heating. For example three winter days in a row with average temperatures of 4°C, -2°C and -4°C totals to 56 HDD. Accordingly, three summer days of 26°C, 28°C, and 30°C totals to 30 CDD.

Monitoring heating degree day information over time, relative to your fuel consumption, can provide a method of evaluating the impact of energy conservation practices that you may choose to try.