Degree Days Calculator
Heating and cooling degree days for a period, and the energy a building of a given fabric would need.
Results
What this tool does
Degree days measure how cold a period was and for how long, in one number, which is what makes energy comparisons fair across winters. The energy figure here covers fabric heat loss only: it ignores ventilation, solar gain, internal gains from people and appliances, and boiler efficiency, all of which are large.
Formula
degrees-day = days × (T base − T average)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tb | Base temperature | °C |
tm | Mean outdoor temperature | °C |
dd | Days | — |
uv | U-value | W/(m²·K) |
ar | Area | m² |
HD | Degree days | °C·d |
CD | Degree days | °C·d |
EN | Energy | kWh |
DA | Degree days | °C·d |
Worked example
- Base temperature15.5 °C
- Mean outdoor temperature8 °C
- Days31
- U-value0.3 W/(m²·K)
- Area200 m²
- Degree days232.500 °C·d
- Degree days0.000 °C·d
- Energy334.80 kWh
- Degree days7.500 °C·d
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
What are degree days actually for?
For comparing energy use between periods with different weather. A building that burned more gas this January than last may simply have had a colder January; dividing consumption by degree days removes that and shows whether the building itself changed. The base temperature is the outdoor temperature below which heating is needed, and it depends on the building — 15.5 °C is a long-standing convention for older housing, but a well insulated one may not need heating until 12 or lower.