Heating and Cooling Degree Days Calculator
Degree days measure how much, and for how long, the outside temperature differs from a comfortable base, and they are the standard way to relate weather to building energy use. Heating degree days (HDD) accumulate when it is colder than the base and a building needs heating; cooling degree days (CDD) accumulate when it is warmer and needs cooling. The base temperature is the outdoor level at which a building needs neither, commonly 18 degrees Celsius or 65 degrees Fahrenheit. This tool computes HDD and CDD for a day from the mean temperature and base, and scales them over a number of days. The base temperature is user-editable so you can match your reference.
Degree days formula
HDD per day = max(0, base - mean temperature)
CDD per day = max(0, mean temperature - base)
total = degree days per day * number of days
For each day, if the mean temperature is below the base the difference counts as heating degree days; if above, the difference counts as cooling degree days. A day is never both, and a day at the base counts zero. The mean is usually taken as the average of the daily maximum and minimum. Multiplying the daily figure by the number of days estimates the total for a billing period or season; for real records you would sum each day individually.
Worked example
With a mean temperature of 10 and a base of 18 (same units), HDD = max(0, 18 - 10) = 8.00 per day and CDD = max(0, 10 - 18) = 0.00. Over 30 such days the total is 8 * 30 = 240.00 heating degree days and 0.00 cooling degree days. A warmer mean of 25 against the same base would instead give 7.00 cooling degree days per day and no heating degree days.
Frequently asked questions
What is a degree day?
A degree day is one degree of temperature difference from a base, sustained for one day. If the mean temperature is five degrees below the base, that day contributes five heating degree days. Summing degree days over a period gives a single number that captures both how cold (or warm) it was and how long, which correlates closely with heating or cooling energy use.
What base temperature should I use?
The base is the outdoor temperature at which a building needs no heating or cooling, set by its insulation, internal gains, and comfort target. Common defaults are 18 degrees Celsius or 65 degrees Fahrenheit, but the right base varies by building. The base is user-editable; for energy analysis, choose the base that best fits your building's balance point.
How are degree days used?
Utilities and energy analysts use degree days to normalise energy bills for weather, compare consumption between periods, size heating and cooling plant, and estimate seasonal demand. Because degree days track the driver of thermal energy use, they let you separate weather effects from changes in efficiency or occupancy.
Can a single day be both heating and cooling?
With the simple mean-temperature method used here, no: a day is classed by whether its mean is above or below the base, giving either heating or cooling degree days but not both. More detailed methods that look at hourly temperatures can register some heating and some cooling on the same day, for example a cold morning and a warm afternoon.
Official sources
- NOAA National Centers for Environmental Information: degree days and climate data.
- U.S. Energy Information Administration: degree days explained.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.