Key takeaways
- A day with a mean temperature of 12 °C adds 6 heating degree days against an 18 °C base; days on the other side of the base count as zero.
- The base temperature is a choice. 18 °C and 65 °F (18.3 °C) are common conventions, but a building's own base is often lower because people and equipment provide free heat.
- Bases differ between users: ASHRAE 169 zones use HDD18 and CDD10, Eurostat counts HDD on days at or below 15 °C against 18 °C, and NOAA uses 65 °F for both.
- Calculating from hourly data (the integration method) is more accurate than using daily means, especially on days that cross the base.
- Degree days only see air temperature. They ignore sun, humidity and wind, so they suit comparison and screening, not system sizing.
What are heating and cooling degree days?
A degree day is a sum of temperature differences. For each day, heating degree days (HDD) are the number of degrees by which the mean outdoor temperature falls below a base temperature, and cooling degree days (CDD) the number by which it rises above one. Days on the other side of the base count as zero, and the daily values are added up over a month or a year.
The idea is that, all else being equal, the energy needed to heat a building is roughly proportional to heating degree days: a winter with 10% more heating degree days needs about 10% more heating energy.
What base temperature should you use?
The base temperature is the outdoor temperature at which a building no longer needs heating (or starts to need cooling). It sits below the indoor set point because people and equipment provide free heat. An office heated to 20 °C with about 3 °C of internal gains, for example, has a heating base temperature of around 17 °C.
Published degree days use conventions rather than building-specific values:
- 65 °F (18.3 °C): NOAA's Climate Prediction Center uses it as the base for both heating and cooling degree days in the United States.
- HDD18 and CDD10: the bases ASHRAE Standard 169 uses to assign climate zones 0 to 8.
- Eurostat: heating degree days are counted on days with a mean at or below 15 °C, as 18 °C minus the mean; cooling degree days on days at or above 24 °C, as the mean minus 21 °C.
- 15.5 °C: the heating base traditionally quoted in the UK.
How do you calculate degree days?
The simplest method uses the daily mean temperature, often taken as the average of the day's maximum and minimum:
- Find the daily mean: a maximum of 15 °C and a minimum of 5 °C give a mean of 10 °C.
- For heating, subtract the mean from the base: with an 18 °C base that day adds 18 − 10 = 8 HDD, and no CDD.
- Repeat for every day and add up: a 30-day month in which every day averages 10 °C accumulates 240 HDD.
- For cooling, subtract the base from the mean on warm days: a day averaging 26 °C adds 26 − 18.3 = 7.7 CDD at an 18.3 °C base.
- To convert Fahrenheit-based degree days to Celsius, multiply by 5/9 without subtracting 32, because degree days are temperature differences.
Why is the hourly method more accurate?
The integration method works from hourly or more frequent readings and adds up the area between the temperature curve and the base through the day. It matters on days that cross the base. A mild day with a cold night can have a mean equal to the base and score zero with the daily-mean method, even though a building would need heating overnight and perhaps cooling in the afternoon.
An hourly weather file such as an EPW makes this easy: add up (base minus temperature) for every hour below the base and divide by 24 to get heating degree days, and do the reverse for cooling.
Where are degree days used, and what are their limits?
- Climate classification: ASHRAE 169 thermal zones are defined by HDD18 and CDD10, and those zones select requirements in ASHRAE 90.1 and the IECC.
- Energy statistics: Eurostat publishes monthly heating and cooling degree days for EU regions from 1975 onwards to track weather-driven demand.
- Monitoring: dividing a building's metered heating energy by degree days separates the effect of the weather from the effect of efficiency measures.
- Climate change: Belcher et al. (2005) checked their future weather files by showing that the heating degree days calculated from them fell by about as much as those calculated directly from the climate model.
- Limit: degree days use air temperature only, so they ignore solar gains, humidity and wind; two climates with the same CDD can have very different cooling loads if one is humid.
- Limit: they assume heating or cooling energy is proportional to the difference from one fixed base, which only approximates how real buildings respond, and the result depends heavily on that base.
- Limit: they are not a sizing tool. Peak loads depend on design conditions and hour-by-hour behaviour, not on annual totals.
Degree days in C4B
C4B shows heating and cooling degree days at an 18.3 °C base for every location, next to its Köppen-Geiger and ASHRAE 169 climate zones. It does not have a separate degree-day module. For a building-specific base temperature, work from the hourly weather data, or use C4B's preliminary energy balance, which calculates heating and cooling hour by hour.
Frequently asked questions
What base temperature should I use for heating degree days?
There is no single right answer. For comparing climates, use one convention consistently, such as 18 °C or 65 °F. For analysing a particular building, use a base close to the outdoor temperature at which that building stops needing heat, which is usually a few degrees below its heating set point.
How do I convert Fahrenheit degree days to Celsius?
Multiply by 5/9 and convert the base temperature as well: 900 degree days at a 59 °F base equal 500 degree days at a 15 °C base. Do not subtract 32, because degree days measure temperature differences.
Can a day have both heating and cooling degree days?
With the daily-mean method and one base temperature, no. With hourly calculations it can: a cold night can add heating degree days and a hot afternoon cooling degree days. With separate thresholds, such as Eurostat's 15 °C and 24 °C, a mild day can add neither.
Are degree days enough to estimate a building's energy use?
They are enough for a first comparison of climates, or for tracking one building's consumption from year to year. For design decisions, use an hourly simulation, which accounts for sun, humidity, thermal mass and occupancy.
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References
- Eurostat: Cooling and heating degree days, reference metadata
- NOAA Climate Prediction Center: Degree days explanation
- Degree Days.net: Degree days, an introduction
- Degree Days.net: Degree-day calculation methods
- ICC: 2021 International Energy Conservation Code, Table C301.3 thermal climate zone definitions
- Belcher, Hacker and Powell (2005), Constructing design weather data for future climates
Related guides
ASHRAE 169 climate zones
How ASHRAE 169 thermal zones and moisture regimes are defined, how to find a site's zone and where the zones are used.
EPW weather files
What is inside an EPW weather file, where the data comes from, and how to open, check and use one on a project.
Typical meteorological year (TMY)
How typical meteorological years are selected, how TMY, TMYx and ISO 15927-4 reference years differ, and when to use actual or extreme years instead.
Future weather files and morphing
How morphing combines an observed weather year with climate model projections, how the methods differ and what future weather files can and cannot tell you.