Degree day calculator

How it works

Formula: Heating = base − daily mean (if positive)

A day's degree days measure the gap between its mean temperature and a base temperature. The National Weather Service uses 65 °F: heating degree days are 65 minus the mean, cooling degree days the mean minus 65. The daily mean is the high plus the low divided by two. Environment Canada uses 18 °C, and growing degree days above 5 °C. Add up the days of a period.

• Environment and Climate Change Canada — Glossary (Degree-days, Heating and Cooling Degree-days)
• National Weather Service Key West — What Are Heating and Cooling Degree Days

Worked examples

• A day with a 50 °F mean

  15 heating degree days.

• A week of 35, 40, 45, 50, 42, 38 and 33 °F

  172 heating degree days.

• A day with an 85 °F mean

  20 cooling degree days.

Common mistakes

• Counting negative degree days.

  A day warmer than the base gives 0 heating degree days, not a negative number.

• Averaging degree days instead of adding them.

  A period's degree days add up day by day.

• Using the afternoon temperature.

  Use the daily mean: (high + low) ÷ 2.

What the calculation leaves out

• Degree days give an idea of heating or cooling needs, not a building's exact energy use.
• For plant growth, the base varies by crop; the tool lets you change it.

Degree days of one day by its mean temperature

Frequently asked questions