Degree Days Calculator (Heating/Cooling)

A degree day measures how cold or warm a location is relative to a standard base temperature — typically 65°F (18°C) in the United States. It compares the mean outdoor temperature for a day to that base. The more extreme the outside temperature, the higher the number of degree days, and generally the higher the energy use for space heating or cooling.

Heating Degree Days (HDD) measure how cold a day was — they indicate how much energy may be needed for heating. A day with a mean temperature of 40°F has 25 HDDs (65 - 40 = 25). Cooling Degree Days (CDD) measure how hot a day was and indicate cooling energy demand. A day with a mean of 80°F has 15 CDDs (80 - 65 = 15). A single day can only contribute to HDD or CDD, not both.

65°F (18.3°C) has historically been used as the standard base because most buildings require neither heating nor cooling when the mean outdoor temperature is at or near 65°F. However, modern buildings with better insulation may use different base temperatures (e.g. 60°F), and utility companies may specify their own base. Always match the base temperature used by your energy provider for accurate bill reconciliation.

Energy professionals use degree day data to normalize energy consumption across seasons and years, making it possible to compare energy efficiency independently of weather. Common applications include tracking heating fuel consumption, estimating HVAC system performance, benchmarking buildings, planning energy audits, and predicting future energy bills.

To calculate degree days for a period, you sum the daily degree days for each day in that period. This calculator lets you enter a number of days to quickly estimate cumulative HDD or CDD when daily temperatures are consistent. For precise monthly or annual totals, use daily temperature records and sum each day's contribution individually.

Divide your actual energy consumption (e.g. gallons of oil or kWh) by the number of degree days in the billing period to get a consumption-per-degree-day figure. Compare this across billing periods to identify changes in efficiency. Make sure you use the same base temperature as your utility provider — often this information is printed on your bill or available from your supplier.

Population-weighted degree days adjust the regional average degree day figure based on where people actually live within a region. Areas with higher population density contribute more to the average, making the metric more representative of actual energy demand across a populated area. The U.S. Energy Information Administration (EIA) uses population-weighted degree days in national energy reports.

Yes. In metric countries, degree days are calculated in Celsius, often with a base temperature of 15.5°C (equivalent to 60°F) or 18°C (equivalent to 64.4°F). The formula is identical — subtract the mean daily temperature from the base (for HDD) or vice versa (for CDD). This calculator supports both Fahrenheit and Celsius inputs.