Cooling Degree Days
Calculator
Results
- Cooling degree days (°C·day)
- 240
- Degree days per day
- 8
- Cooling degree days (°F·day)
- 432
Weather results
| Cooling degree days (°C·day) | 240 |
| Degree days per day | 8 |
| Cooling degree days (°F·day) | 432 |
formula-map diagram
- Cooling degree days (°C·day)
- 240
- Degree days per day
- 8
- Cooling degree days (°F·day)
- 432
Weather relationship
Formula
CDD = Σ max(0, T_mean − T_base) over the period (mean-temperature method)= 240
Note
This is a simplified model: it applies the published standard formula to the numbers you entered and ignores local terrain, radiation, precipitation type and instrument error. Wind chill is the 2001 NWS/Environment Canada regression, valid at or below 10 °C with wind at or above 4.8 km/h; the heat index is the Rothfusz regression, reliable above about 27 °C; dew point uses the Magnus approximation (a = 17.27, b = 237.7 °C) over liquid water; wet bulb uses Stull's approximation at standard sea-level pressure; pressure and density altitude assume the International Standard Atmosphere and dry air. Do not use these results for aviation, structural or safety decisions without an official forecast or a qualified professional.
More in Weather and climate
See all →Frequently asked questions
What are cooling degree days (CDD) measuring?+
CDD measures cumulative demand for air conditioning over a period by summing, for each day, how many degrees the average temperature exceeded a reference baseline (commonly 18°C or 65°F), then adding up those daily excesses across the period.
How is a single day's CDD contribution calculated?+
Take that day's average temperature, subtract the baseline temperature, and if the result is positive, that's the day's CDD value; if the average temperature is at or below the baseline, that day contributes zero, since no cooling demand is assumed on a cool day.
Why is 18°C (65°F) commonly used as the baseline?+
That baseline reflects a temperature at or below which most buildings need little to no active cooling to stay comfortable; it was established from historical energy-use studies and remains a standard reference even though some regions and modern buildings use a slightly different value.
Why do utility companies and energy analysts track CDD over a season?+
Cumulative CDD correlates strongly with actual air conditioning energy consumption, so it's used to normalize and compare cooling energy usage across different weather years, to forecast electricity demand, and to model how a warmer summer would affect cooling costs.
What's the relationship between cooling degree days and heating degree days?+
They're complementary measures using the same baseline temperature concept: heating degree days sum how far the average temperature falls below the baseline on cold days, while cooling degree days sum how far it rises above the baseline on hot days, and a given day contributes to only one of the two, never both.