EGEE 102
Energy Conservation for Environmental Protection

Heating Degree Days


Local weather or climatic conditions are one of three factors that affects the amount of heat loss through conduction. When examining weather conditions, we look at both the inside and outside temperature of a home.

The inside temperature is usually taken as a standard comfort temperature of 65ºF. The outside temperature varies by the hour. Knowing this information can help us to understand two concepts:

  • Average outside temperature = Average of the maximum and minimum temperature during the day
  • Heating Degree Day (HDD) = The temperature difference through which air has to be treated, or how many degrees the mean temperature fell below 65ºF for the day. It is also an index of fuel consumption.
Image of a house. The inside of the house is 65 degrees. The outside air varies by the hour.
Temperatures Inside and Outside a House

Determining Heating Degree Day (HDD)

The formula for determining the Heating Degree Day (HHD) is:

HDD = T base - T a

To calculate HDD:

  1. Determine the base temperature or inside temperature: T base  = usually 65°F
  2. Find the day's average outside temperature using this formula: T a  = average outside temperature =  T max + T min 2
  3. Use the HDD formula to solve:
    T base  - T a

Note: If the Ta is equal to or above 65 ºF, there are no heating degree days for that 24-hour period, or HDD = 0.

Try This!

Click the play button below, and observe the temperature changes. Then calculate the average temperature and the Heating Degree Day.

Heating Degree Days Activity

Click here to open a text description of the Heating Degree Days activity.

Try This: Determining Heating Degree Day (HDD)

Picture a house with a thermometer on the porch. The current outside temperature is 60 degrees. As thick clouds start to move in, the temperature begins to drop in 5 degree increments, first to 55 degrees, then 50 degrees, etc. When the clouds have completely blocked the Sun, the thermometer reads 40 degrees.


  1. What is the average temperature?
    Hint: Ta = (max temperature + minimum temperature) divided by 2.
  2. What is the heating degree days?
    Hint: HDD = Tbase - Ta


  1. 50 degrees
  2. 15 degrees