Cooling Degree Days Calculator
Cooling Degree Days (CDD) quantify the demand for energy to cool a building. The NOAA standard method uses a base temperature of 65 degrees Fahrenheit: for each day, the mean temperature (average of daily high and low) is compared to 65 F. If the mean exceeds 65 F, that excess is the day's CDD contribution. Values at or below 65 F contribute zero CDD. Annual CDD is the sum of all daily values. CDD are widely used by utilities, engineers, and policymakers to forecast summer cooling energy demand and compare climates. Enter up to 7 days of temperature data below.
NOAA cooling degree days formula
Mean Temp = (Daily High + Daily Low) / 2
CDD(day) = max(0, Mean Temp - Base Temp)
Annual CDD = Sum of CDD(day) for all days in year
This is the method used by NOAA's National Centers for Environmental Information. The base temperature of 65 F was selected because below this temperature most buildings do not require mechanical cooling, as internal heat gains from occupants and appliances are sufficient to maintain comfortable indoor temperatures.
Typical annual CDD values by US city
- Seattle, WA: approximately 250 CDD per year (mild climate, very low cooling demand)
- Chicago, IL: approximately 830 CDD per year (moderate summer cooling demand)
- Dallas, TX: approximately 2,800 CDD per year (hot summers, high cooling demand)
- Phoenix, AZ: approximately 4,000 CDD per year (very high cooling demand)
- Miami, FL: approximately 4,500 CDD per year (near-year-round cooling required)
Cooling degree days calculator: frequently asked questions
What are cooling degree days (CDD)?
Cooling Degree Days measure how much and for how long outside temperatures exceed a base temperature (65 F in the US). Each degree above the base for one day equals one CDD. CDD are used to estimate air conditioning energy demand for buildings.
What base temperature does NOAA use for CDD?
NOAA uses a base temperature of 65 degrees Fahrenheit (18.3 C) for both heating and cooling degree days. When the mean daily temperature exceeds 65 F, the excess represents cooling demand.
How is CDD calculated for a single day?
NOAA calculates mean daily temperature as (High + Low) / 2. If the mean exceeds 65 F, CDD = mean temperature minus 65. If the mean is 65 F or below, CDD = 0 for that day. Annual CDD is the sum of all daily CDD values.
How do CDD relate to air conditioning energy use?
CDD are proportional to cooling energy demand. A location with 2,000 CDD requires roughly twice the air conditioning energy as a location with 1,000 CDD (for the same building and equipment). Utilities and engineers use CDD to forecast summer peak electricity demand.
Where can I get official CDD data for my city?
NOAA's National Centers for Environmental Information (NCEI) publishes monthly and annual CDD data for hundreds of US weather stations at their Climate Data Online portal. Data is available back to the early 1900s for many stations.
Official sources
- NOAA National Centers for Environmental Information: Degree Days (NCEI).
- U.S. Energy Information Administration: Heating and Cooling Degree Days (EIA).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.