Heating and Cooling Degree Days Calculator (HDD, CDD)

What are degree days?

Degree days sum up in one number how much heating or cooling a place needs. For each day you take how far the outdoor temperature is below (heating) or above (cooling) a base temperature, and add the differences up.

Heating degree days (HDD): when the daily mean is below the base temperature, the difference is that day's degree days. With an 18 °C base a day averaging 10 °C counts 8 degree days.

Cooling degree days (CDD): when the daily mean is above the base temperature, the difference counts.

The yearly total shows how severe the climate is. A cold city may have a few thousand heating degree days a year and a mild coastal one a few hundred.

Which base temperature?

There is no single right answer and it depends on the source. Eurostat, followed by the Turkish State Meteorological Service, takes 18 °C as the heating base but counts nothing on days averaging above 15 °C. For cooling, days above 22 °C count in the Turkish study. In the US 65 °F (18.3 °C) is common and in the UK 15.5 °C. To compare with a published figure use the same base and method.

Example

Six days averaging 10, 16, 25, 20, 15 and 22.5 °C. Heating: 8 degree days for 10 °C and 3 for 15 °C. The 16 °C day is above the threshold and counts nothing. HDD totals 11. Cooling: 3 for 25 °C and 0.5 for 22.5 °C, so CDD is 3.5.

Working from monthly means

Without daily data you can approximate with monthly mean temperatures: HDD ≈ days in month × (base − monthly mean). It works well in midwinter and undercounts in shoulder months, because a month whose mean is above the base still contains cold days.

Fuel consumption from degree days

If the building heat loss coefficient H (W/K) is known, annual heat demand is estimated as Q = H × HDD × 24. A house with a heat loss coefficient of 200 W/K in a place with 2000 degree days needs 200 × 2000 × 24 = 9600 kWh of heat. With a 90 percent efficient boiler that is 10 667 kWh of fuel energy, about 1003 m³ of natural gas at 10.64 kWh/m³.

The relation is also useful in reverse: divide last year's bill by that year's degree days and you can see the effect of insulation or new equipment with the weather taken out.

Limits

The degree day method is a rough estimate. Solar gain, internal gains, night setback and thermal mass are not included. Detailed energy calculations use monthly or hourly methods.