Heating Degree Days Calculator
A heating degree days calculator measures how cold a period is and how much heating it is likely to demand. A degree day counts how far the average outdoor temperature falls below a base temperature, usually 65 degrees Fahrenheit, on a given day; summing those differences over many days gives heating degree days, a standard index for comparing climates and estimating heating energy. This tool takes the base temperature, the average outdoor temperature, and the number of days, then multiplies the temperature shortfall below the base by the number of days to return total heating degree days. All three inputs are editable so you can use a different base, model a single month, or compare two locations. Heating degree days scale closely with heating fuel use, so they help you normalize energy bills for weather, size heating equipment, or compare one winter to another. The US Department of Energy and the National Oceanic and Atmospheric Administration publish degree-day data and explain the method. The calculation here treats the average temperature as constant across the days entered; real data sums each day separately. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Heating degree days sum how far temperature falls below the base: HDD = (base - avg temp) x days. A 65 degF base with a 45 degF average over 30 days gives 600 HDD. More HDD means more heating demand.
Heating degree days formula
HDD = (T(base) - T(avg)) x D, when T(avg) is below T(base)
T(base) = base temperature (usually 65 degF)
T(avg) = average outdoor temperature
D = number of days
If the average temperature is at or above the base, no heating degree days accrue for that period. The classic base is 65 degF, the point below which buildings tend to need heat.
Worked example
A 30-day month has an average outdoor temperature of 45 degrees Fahrenheit, compared with a base of 65 degrees Fahrenheit.
- Daily shortfall = 65 - 45 = 20 degree days per day
- Number of days = 30
- Total HDD = 20 x 30 = 600
The month accumulates 600 heating degree days. These are the calculator's default inputs, so the result above matches the widget exactly.
Heating Degree Days Calculator: frequently asked questions
What is a heating degree day?
A heating degree day measures how far the average outdoor temperature falls below a base temperature, usually 65 degrees Fahrenheit, on a single day. If the average is 45 degrees, that day records 20 heating degree days. Summing across days gives the total for a month or season.
Why is the base usually 65 degrees?
The 65 degree Fahrenheit base reflects the temperature below which a typical building needs heating, accounting for heat from people, lights and appliances. Some analyses use a different base to match a specific building, which is why this calculator leaves the base editable.
What are degree days used for?
They normalize energy use for weather. Dividing fuel use by degree days gives a weather-adjusted efficiency figure, so you can compare bills across colder and milder months or years. Degree days also help size heating systems and estimate seasonal heating cost.
Does this sum each day separately?
This calculator multiplies a single average temperature shortfall by the number of days, which is exact when the period truly averages that temperature. Official degree-day totals add each day's value separately, capturing daily swings. For precise figures, use published daily degree-day data.
Where can I get real degree-day data?
The US National Oceanic and Atmospheric Administration and the Department of Energy publish heating and cooling degree-day data by station and region. Use those records for accurate, location-specific totals rather than a single average temperature.
Official sources
- Degree-day method and home energy guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.