Heating degree days are a type of degree day that is used to measure how much (in degrees), and for how long (in days), the outside air temperature was below a certain level. They are used in calculations of the energy consumption required to heat buildings.

The energy consumption by building heating systems is complicated. Essentially, the colder the outside air temperature, the more energy it takes to heat a building. However, the outside temperature does not only vary from one location to another - it also varies through time. If you use your heating system to keep your building at a roughly constant temperature, the amount of energy that your heating system uses will vary, just like the outside air temperature does. Heating degree days are a way to quantify this. The idea is that the amount of energy needed to heat a building in any period (day/week/month/year) is directly proportional to the number of heating degree days in that same period.