Quick answer
Cooling degree days sum how far each day's mean temperature exceeds a base: CDD = Σ max(0, (high + low) ÷ 2 − base). Five cool days with a 65 °F base give 0 CDD over 5 days.
Enter the base temperature and the daily highs and lows. The calculator returns the cooling degree days and days counted.
Related tools: Heating Degree Days Calculator: Heating degree days from daily high and low temperatures. Snow Water Equivalent Calculator: Depth of water in a snowpack from snow depth and density. Heat Index Calculator: Apparent temperature from air temperature and relative humidity (°F).
How to use the Cooling Degree Days Calculator
- Enter base temperature.
- Type daily highs, for example 80, 85, 90.
- Type daily lows, for example 60, 65, 70.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Base temperature | A number | — |
| Daily highs | Text, such as 80, 85, 90 | — |
| Daily lows | Text, such as 60, 65, 70 | — |
Formula and method
Formula
CDD = Σ max(0, (high + low) ÷ 2 − base)
IEEE-754 double precision; shown to the decimals indicated.
Use the same temperature unit for everything; a common base is 65 °F (18 °C).
Worked examples
This example is verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Base temperature = 65; Daily highs = 50, 40, 30, 60, 70; Daily lows = 30, 25, 20, 40, 50.
| Result | Value |
|---|---|
| Cooling degree days | 0 |
| Days counted | 5 |
The first line is the headline result (cooling degree days). Below it the cooling Degree Days Calculator lists days counted, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
How CDD works
A day with a mean of 75 °F against a base of 65 °F adds 10 CDD. Hot summers accumulate hundreds of CDD, which correlate with air-conditioning energy use. Cold days contribute zero.
Base and use
65 °F (18 °C) is the usual base, but buildings with different insulation or internal heat gains use other bases. Utilities use CDD and HDD to normalise energy use for weather. Lists must be the same length.
How the result changes with base temperature
Holding every other input at the first worked example, this table changes only base temperature. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Base temperature | Cooling degree days | Days counted |
|---|---|---|
| 32.5 | 52.5 | 5 |
| 65 | 0 | 5 |
| 130 | 0 | 5 |
| 260 | 0 | 5 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Daily lows: entering "60" shows "Enter one low for every high." beside that field and no result.
- Base temperature: left empty shows "Enter base temperature." beside that field and no result.
Choosing between the Cooling Degree Days Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Heating Degree Days Calculator](/science/heating-degree-days-calculator/) | base temperature, daily highs and daily lows | Heating degree days |
| [Snow Water Equivalent Calculator](/science/snow-water-equivalent-calculator/) | snow depth and snow density | Snow water equivalent |
| [Heat Index Calculator](/science/heat-index/) | air temperature and relative humidity | Heat index |
| [Sunrise and Sunset Calculator](/science/sunrise-sunset-approximation-calculator/) | date, latitude and longitude | Sunrise |
| [Days Until Calculator](/date-calendar/days-until-calculator/) | target date | Days until |
Background: environment and acoustics
Decibels are a logarithmic scale, so sound levels cannot be added arithmetically: two equal sources raise the level by about 3 dB, not double it. Environmental and emissions results depend on the factors behind them (for example the US EPA's equivalency factors), which are averages for a country, fuel and year.
Use the result to compare scenarios rather than as a measured value.
Assumptions and limitations
- Lists must be the same length
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for daily lows and base temperature to confirm this.
Frequently asked questions
What is a heating degree day?
The reverse measure for cold days.
Why zero for a cool week?
No mean temperature exceeded the base.
What base should I use?
65 °F in the US, 15.5 or 18 °C elsewhere.
What does the Cooling Degree Days Calculator calculate?
Cooling degree days from daily high and low temperatures. It returns Cooling degree days and Days counted.
What formula does the Cooling Degree Days Calculator use?
CDD = Σ max(0, (high + low) ÷ 2 − base). Use the same temperature unit for everything; a common base is 65 °F (18 °C).
What do I need to enter in the cooling Degree Days Calculator?
You enter Base temperature, Daily highs and Daily lows.
What is an example of a cooling Degree Days result?
With base temperature 65, daily highs 50, 40, 30, 60, 70 and daily lows 30, 25, 20, 40, 50, the cooling degree days is 0.
Where does the method come from?
The method follows: Standard degree-day definition (NOAA / ASHRAE).
Related calculators
- Heating Degree Days Calculator — Heating degree days from daily high and low temperatures.
- Snow Water Equivalent Calculator — Depth of water in a snowpack from snow depth and density.
- Heat Index Calculator — Apparent temperature from air temperature and relative humidity (°F).
- Sunrise and Sunset Calculator — Approximate sunrise, sunset, solar noon and day length for a date and location.
- Days Until Calculator — Days remaining until any date (generalizes “days until June 15”, “days until Christmas” and similar pages).
- Days Old Calculator — How many days old you are.
Sources
- Standard degree-day definition (NOAA / ASHRAE)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.