Calculate heating degree days from daily temperatures. Estimate HDD for energy analysis, fuel planning, and heating season comparisons.
Heating Degree Days (HDD) measure how much and for how long the outdoor temperature was below a base temperature (typically 65°F). Each degree below the base for one day equals one HDD. If the average daily temperature is 40°F, that day contributes 25 HDD.
HDD is the foundation of energy analysis. Buildings need heating in proportion to HDD: a city with 7,000 HDD needs roughly twice the heating energy as one with 3,500 HDD. Energy planners, utilities, and auditors use HDD to normalize fuel consumption, predict demand, and evaluate efficiency improvements.
This calculator computes HDD from daily high and low temperatures. Enter temperatures for individual days or use average monthly values to estimate monthly or seasonal HDD totals.
Integrating this calculation into regular energy reviews ensures that conservation strategies are grounded in measured data rather than assumptions about building performance and usage patterns. Precise measurement of this value supports sustainable energy planning and helps organizations reduce their environmental impact while maintaining operational performance and comfort levels.
HDD lets you compare heating energy use between different years, seasons, and locations on an apples-to-apples basis. A warm winter needs fewer HDD, explaining lower fuel bills without any efficiency change. This quantitative approach replaces rough estimates with precise figures, enabling facility managers to identify the most cost-effective opportunities for reducing energy consumption.
Daily HDD = max(0, Base Temperature − Average Daily Temperature) Average Daily Temperature = (High + Low) / 2 Monthly HDD = Daily HDD × Number of Days
Result: 930 HDD for the month
January with average temperature of 35°F and base of 65°F: Daily HDD = 65 − 35 = 30. Monthly HDD = 30 × 31 days = 930 HDD.
Climate Zone 1 (hot): 0–2,000 HDD. Zone 2: 2,000–3,000. Zone 3: 3,000–4,000. Zone 4: 4,000–5,000. Zone 5: 5,000–6,000. Zone 6: 6,000–7,000. Zone 7: 7,000–9,000. These ranges determine building code insulation requirements. Higher HDD = more insulation needed.
Energy managers use HDD to "weather-normalize" consumption. If this winter had 10% fewer HDD than last winter, a 10% decrease in fuel use is expected, not an efficiency improvement. True savings exist only if fuel per HDD decreases.
Superinsulated homes and buildings with high internal gains don't need heating until outdoor temps drop well below 65°F. Using a 55°F or 60°F base temperature for HDD gives more accurate predictions for these buildings. The optimal base temperature can be determined by correlating actual fuel use with HDD at different bases.
Internal heat gains from people, lights, and appliances typically warm a building 5–10°F above outdoor temperature. With a 70°F thermostat, heating is needed when outdoor temperature drops below about 65°F. Well-insulated homes may use a 60°F base.
Divide heating fuel consumption by HDD to get fuel per degree day. If you used 800 therms in a 6,000 HDD winter and 700 therms in a 5,500 HDD winter, normalized usage is 0.133 vs 0.127 therms/HDD — slight improvement.
HDD counts degrees below the base (heating needed). CDD counts degrees above the base (cooling needed). Both use 65°F as the standard base. A 90°F day has 25 CDD. A 30°F day has 35 HDD.
Yes, approximately. Annual Cost ≈ UA × HDD × 24 / (efficiency × 100,000) × fuel price. You need to know your building's heat loss coefficient (UA), but once calibrated, HDD predictions are within 10–20% of actual costs.
NOAA provides free historical HDD data at weather.gov. You can also use degreedays.net for custom base temperatures and date ranges. Your utility bill may include HDD for the billing period.
Sum the daily HDD for all 365 days: HDD_annual = Σ max(0, 65 − T_avg) for each day. Or use monthly averages multiplied by days per month as a reasonable approximation.