Heating Degree Days (HDD) Calculator

About the Heating Degree Days (HDD) Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the base temperature (usually 65°F).
2. Enter the average daily temperature (or high and low for automatic averaging).
3. Enter the number of days at that temperature.
4. The calculator computes total HDD for the period.
5. Repeat for additional months or periods as needed.

Example Calculation

Tips & Best Practices

• The standard base temperature is 65°F (18.3°C) in the US.
• A lower base temperature (e.g., 60°F) can be used for well-insulated buildings with high internal gains.
• Typical annual HDD: Minneapolis 7,800; Chicago 6,500; New York 4,800; Atlanta 2,800; Miami 200.
• Compare year-to-year fuel use by dividing by HDD — this normalizes for weather differences.
• Cooling degree days (CDD) use the same concept but above the base temperature.
• HDD data is available free from NOAA at weather.gov.

HDD by US Climate Zone

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.

Weather Normalization

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.

Variable Base Degree Days

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.

Frequently Asked Questions

• Why is 65°F used as the base temperature?

  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.

• How do I use HDD to compare heating bills?

  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.

• What is the difference between HDD and CDD?

  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.

• Can HDD predict my heating bill?

  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.

• Where can I find HDD data for my location?

  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.

• How do I calculate HDD for a whole year?

  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.