A measure of the deviation of daytime temperature from a predefined standard. This measurement is commonly used to calculate heating and cooling loads as seasons change. Average daytime temperature is subtracted from 65°F (18°C), and the result produces a heating degree day or cooling degree day value. Negative values are expressed as heating degree days; positive values as cooling degree days. For example, if the average temperature in a given week is 50°F, subtraction yields a value of -15. When that figure is multiplied by the seven days in a week, the result is 75 heating degree days.

Degree days are measured over extended periods to assist in estimating power and energy costs, but each day is calculated separately. If mean daytime temperature for a whole year is 64°F, it doesn't produce a value of 365 heating degree days, because each day that heating or cooling is required adds to the total number of degree days for the year.

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