Formula & worked example
HDD=max(0,Tbase−Tmean)·days; CDD=max(0,Tmean−Tbase)·days.
Mean 10°C against an 18°C base for 30 days gives 240 °C-days HDD.
Heating & HVAC engineering calculator · Updated Sep 17, 2026
Heating Degree Days Calculator — HDD, CDD & Base Temperature
Calculate heating and cooling degree days from mean outdoor temperature, balance-point temperature and period length in °C-day and °F-day.
Inputs
Live result
Heating degree days 240 °C·day (432 °F·day)
Cooling degree days 0 °C·day (0 °F·day)
Dominant mode heating
Assumptions & limits
- Uses a period mean; daily weather data gives a more accurate aggregate when temperatures cross the base.
- Degree days indicate weather demand, not building efficiency by themselves.
Frequently asked questions
What does this Heating Degree Days Calculator calculate?
Calculate heating and cooling degree days from mean outdoor temperature, balance-point temperature and period length in °C-day and °F-day.
What units are supported?
Inputs state their unit basis and results include practical SI and US customary equivalents where useful, so the calculation can be checked against field data and equipment schedules.
Can I use this for final equipment selection?
Use it for transparent first-pass sizing and engineering cross-checks. Final selections should use measured conditions, manufacturer performance data and applicable codes or design standards.