Degree Days
page last updated: December 5, 2025
Definitions
- A Heating Degree Day (HDD) is a measure of how cold the weather is — used to estimate heating energy demand.
- Heating Degree Days tell how much (and for how long) the outside temperature was below a certain base temperature (65°F).
- A Heating Degree Day for a given day is calculated as: HDD=65°F-Average Daily Temperature
- A Cooling Degree Day (CDD) measures how warm the weather is and estimates the demand for cooling (air conditioning).
- Cooling Degree Days tell how much (and for how long) the outside temperature was above a certain base temperature (65°F).
- A Cooling Degree Day for a given day is calculated as: CDD=Average Daily Temperature - 65°F
Source: U.S. National Oceanic and Atmospheric Administration
Degree Days
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