How to Calculate Degree Days
What is Degree Days?
Heating and cooling degree days quantify heating/cooling needs based on temperature deviation from base (usually 65°F/18°C).
Formula
Heating degree days (HDD) = sum of (65 - daily avg temp) for days below 65°F
- HDD
- sum of (65 - daily avg temp) for days below 65°F — sum of (65 - daily avg temp) for days below 65°F
- F
- F value — Variable used in the calculation
Step-by-Step Guide
- 1Heating degree days (HDD) = sum of (65 - daily avg temp) for days below 65°F
- 2Cooling degree days (CDD) = sum of (daily avg - 65) for days above 65°F
- 3Higher values indicate greater heating/cooling needs
Worked Examples
Input
City: 5000 HDD/year (cold climate), another 1000 (warm)
Result
Cold climate needs 5× more heating, affecting system size and fuel use
Major design driver
Common Mistakes to Avoid
- ✕Assuming heating and cooling degree days sum equally (different impacts)
- ✕Using wrong base temperature (varies by building type)
Frequently Asked Questions
Why use degree days instead of average temperature?
Degree days directly correlate with energy use; capture heating/cooling needs more accurately.
Can you estimate energy use from degree days?
Roughly yes; heating energy ≈ HDD × building UA / efficiency. Useful for screening.
Ready to calculate? Try the free Degree Days Calculator
Try it yourself →