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What is a Cooling Degree Day?

Ken Black
Ken Black

A cooling degree day (CDD), also known as cooling degree days, is a formula measuring how hot it has been relative to a base temperature over a 24 hour period. The greater the number cooling degree day measurement, the more the air conditioner is likely to be running. Similar in concept is that of heating degree days. The main difference, of course, being that with heating degree days the furnace is the device expected to be working additional time.

The way to find out the cooling degree day number is to use the high temperature and add it to the low temperature for the day in question. that number is then divided by two. For example, if the high one day is 90 degrees Fahrenheit (32 Celsius), and the low is 65 Fahrenheit (18 Celsius), the average temperature would be 77.5 Fahrenheit (25 Celsius). This average Fahrenheit temperature would then be subtracted from 65 Fahrenheit (18 Celsius). Although it is possible to calculate a cooling degree day from any temperature, this one was chosen as a comfortable middle. The result would be 12.5 cooling degree days for that particular day.

A programmable thermostat.
A programmable thermostat.

However, there are other conditions which may make an air conditioner work harder on certain days. It is impossible to say for sure what the cooling degree day measurement may mean without noting these other circumstances. Factors such as the heat index, which includes not only temperature but humidity, the time the furnace is running, whether the thermostat is programmable and a family's individual preference for a certain temperature all play a role. The important thing to remember is that a cooling degree figure is only a guideline to help plan for energy usage.

Cooling degree days are calculated based on the average outdoor temperature.
Cooling degree days are calculated based on the average outdoor temperature.

Still, for those who have been in a home for a while and are familiar with their air conditioning units, a cooling degree day number can provide a baseline figure for which to predict energy usage. This is especially true for those families whose habits or situations remain steady throughout most of the warmer months. While it may be possible to do this simply by looking at high and low temperatures, this formula provides an easy baseline and is fairly simple to remember.

This can be especially important for people who want to make sure they are budgeting enough money each month for that next month's energy bill. Many times, utility bills can fluctuate wildly during warmer months as some months will be hotter than others. Keeping track of cooling degree days should help those who want a more accurate measure of what they will facing.

Discussion Comments


And then, there are people for whom a cooling degree day means precisely nothing. My aunt only turns the air conditioning on when it's 95 degrees outside. Otherwise, she keeps the windows wide open and everyone in the house sweats. I'm not that way. I keep the house about 72 year-round.


This is one of those meteorology terms you don't hear very much anymore. I think it's probably because most people didn't understand how it was calculated.

The heat index number is much easier to understand, and people seem to relate to it as well as they do wind chill factor.

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    • A programmable thermostat.
      By: Bill Bradford
      A programmable thermostat.
    • Cooling degree days are calculated based on the average outdoor temperature.
      By: Silkstock
      Cooling degree days are calculated based on the average outdoor temperature.