Heating degree-days for a given day are the number of degrees Celsius that the mean temperature is below 18 °C. If the temperature is equal to or greater than 18 °C, then the number will be zero. For example, a day with a mean temperature of 15.5 °C has 2.5 heating degree-days; a day with a mean temperature of 20.5 °C has zero heating degree-days. Heating degree-days are used primarily to estimate the heating requirements of buildings.