Degree-days: heating, cooling and growing
Degree-days measure how far a day's mean temperature is from a base. Environment Canada uses 18 °C for heating: a day averaging 15.5 °C counts 2.5 heating degree-days.
Updated November 7, 2026 · Sources
How they are counted
Take the mean temperature of the day, usually the average of its minimum and maximum: 8 °C and 16 °C give 12 °C. Heating degree-days are the base minus that mean; cooling degree-days, the mean minus the base. A negative result counts as zero.
Environment Canada uses 18 °C for heating and cooling, and 5 °C for growing degree-days. The US National Weather Service uses 65 °F. A day averaging 25 °C counts 7 cooling degree-days.
Degree-days by daily mean
Degree-days of one day, by its mean temperature, with Environment Canada's bases. Over a month or a season, add up the days.
| Daily mean | Heating (18 °C) | Cooling (18 °C) | Growing (5 °C) |
|---|---|---|---|
| -10 °C | 28 | 0 | 0 |
| -5 °C | 23 | 0 | 0 |
| 0 °C | 18 | 0 | 0 |
| 5 °C | 13 | 0 | 0 |
| 10 °C | 8 | 0 | 5 |
| 15 °C | 3 | 0 | 10 |
| 18 °C | 0 | 0 | 13 |
| 20 °C | 0 | 2 | 15 |
| 25 °C | 0 | 7 | 20 |
| 30 °C | 0 | 12 | 25 |
What they are used for
- Heating degree-days follow the heating needs of a building: twice as many usually means about twice the heat.
- Cooling degree-days do the same for air conditioning.
- Growing degree-days follow the development of crops and plants, which only grow above a certain temperature.