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Heating & Cooling Degree Days Calculator

Calculate heating degree days (HDD) and cooling degree days (CDD) from temperature data. Estimate annual energy consumption for buildings.

The Heating & Cooling Degree Days Calculator computes HDD and CDD from temperature data to estimate building energy consumption. Degree days quantify heating and cooling demand based on outdoor temperature deviation from a base temperature.
Temperature Data Input
Typical base temperature: 65°F (18°C) for heating, 65°F for cooling in US. Energy estimates are approximations - actual consumption depends on building efficiency, HVAC system, and occupant behavior.

What are Heating & Cooling Degree Days?

Degree days are a measure of how much (and for how long) outdoor temperature deviates from a comfortable base temperature. Heating Degree Days (HDD) accumulate when temperatures fall below the base, indicating heating is needed. Cooling Degree Days (CDD) accumulate when temperatures rise above the base, indicating cooling is needed. Degree days correlate strongly with energy consumption and are used for: energy benchmarking, predicting utility bills, comparing buildings across climates, and sizing HVAC equipment. The standard base temperature is 65°F (18°C) in the US, assuming buildings don't need heating/cooling when outdoor temps are near 65°F.

Degree Day Formulas

2. Heating Degree Days (HDD)

HDD = max(0, Tbase - Tavg)

Example: Base 65°F, Avg 45°F → HDD = 65 - 45 = 20 HDD

3. Cooling Degree Days (CDD)

CDD = max(0, Tavg - Tbase)

Example: Base 65°F, Avg 80°F → CDD = 80 - 65 = 15 CDD

Practical Tips

  • Use weather station data close to your building location for accuracy
  • 65°F/18°C base works for most buildings, but adjust for high internal gains
  • Degree days alone don't predict energy - building efficiency matters greatly
  • HDD/CDD correlate best with gas heating and electric AC consumption
  • For energy prediction: Energy ≈ HDD × Heat Loss Factor (Btu/HDD)
  • Historical 30-year average degree days used for 'typical year' analysis
  • Use actual year degree days to explain bill variations year-to-year
  • Combine with building square footage to calculate Energy Use Intensity (EUI)
See also
HVAC CalculatorsHVAC CALCULATORS6
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