Heating Degree Days Calculator
Heating Degree Days (HDD) quantify the demand for energy to heat 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 subtracted from 65 F, and if the result is positive that value is the day's HDD contribution. Negative values are set to zero. Annual HDD is the sum of all daily values. Enter up to 7 days of temperature data below, or use the single-day calculator for a quick estimate.
NOAA heating degree days formula
Mean Temp = (Daily High + Daily Low) / 2
HDD(day) = max(0, Base Temp - Mean Temp)
Annual HDD = Sum of HDD(day) for all days in year
This is the method used by NOAA's National Centers for Environmental Information (NCEI). The base temperature of 65 F was established by the American Gas Association and adopted by NOAA based on empirical observation that below 65 F, most US residential buildings require mechanical heating.
Typical annual HDD values by US city
- Miami, FL: approximately 150 HDD per year (very warm climate, minimal heating)
- Atlanta, GA: approximately 2,800 HDD per year (mild heating season)
- Chicago, IL: approximately 6,100 HDD per year (significant heating demand)
- Minneapolis, MN: approximately 7,800 HDD per year (one of the coldest major US cities)
- Anchorage, AK: approximately 10,500 HDD per year (very high heating demand)
Heating degree days calculator: frequently asked questions
What are heating degree days (HDD)?
Heating Degree Days measure how much and for how long outside temperatures fall below a base temperature (65 F in the US, 15.5 C in the UK). Each degree below the base for one day equals one HDD. HDD are used to estimate heating energy demand for buildings.
What base temperature does NOAA use for HDD?
NOAA uses a base temperature of 65 degrees Fahrenheit (18.3 C). This is the outdoor temperature below which most buildings require heating to maintain a comfortable indoor temperature of around 70 F, accounting for internal heat gains from occupants and appliances.
How does NOAA calculate HDD for a single day?
NOAA calculates the mean daily temperature as (High + Low) / 2. If the mean is below 65 F, HDD = 65 - mean temperature. If the mean is 65 F or above, HDD = 0 for that day. Annual HDD is the sum of all daily HDD values.
Where can I find official HDD data for my city?
NOAA publishes official monthly and annual HDD data at the National Centers for Environmental Information (NCEI) Climate Data Online portal (ncdc.noaa.gov). Historical daily normals are available for thousands of US weather stations.
How are HDD used in energy calculations?
HDD are directly proportional to heating energy demand. Doubling the HDD roughly doubles the heating fuel needed to maintain the same indoor temperature, assuming constant building envelope performance. This makes HDD a key input for sizing heating systems and estimating fuel costs.
Official sources
- NOAA National Centers for Environmental Information: Climate at a Glance: Degree Days.
- NOAA Technical Documentation on Degree Days: Degree Days (NCEI).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.