Degree Days Formula & How to Calculate It

To calculate degree days, subtract the base temperature from the daily mean temperature; if the result is positive, that number is the degree day value for that day. You compute the daily mean by averaging the day’s high and low temperatures, and you choose a base temperature that suits your purpose (commonly 65°F for energy use or 50°F for plant growth). This article explains the formula, gives worked examples, and covers the most common uses and pitfalls.

Formula

The basic degree day formula is:

• Heating degree days (HDD) = max(0, T_base − T_mean)
• Cooling degree days (CDD) = max(0, T_mean − T_base)

Where:

• T_mean = (T_high + T_low) / 2, the daily mean temperature (typically in °F or °C).
• T_base = the reference temperature below which (for heating) or above which (for cooling) energy is needed. Common values are 65°F (18°C) for building energy and 50°F (10°C) for crop growth.

The “max(0, …)� simply means that if the result is negative, you record zero degree days for that day—no heating or cooling is needed.

Worked Example

Using the calculator defaults: high = 75°F, low = 55°F, base = 65°F.

1. Calculate the daily mean: (75 + 55) / 2 = 130 / 2 = 65°F.
2. Heating degree days: max(0, 65 − 65) = 0 HDD.
3. Cooling degree days: max(0, 65 − 65) = 0 CDD.

So with a mean exactly equal to the base, you get zero degree days. Try a colder day: high = 55°F, low = 35°F. Mean = (55 + 35) / 2 = 45°F. HDD = 65 − 45 = 20 heating degree days. CDD = 0.

Why Use Degree Days?

Degree days are a simple way to quantify how much heating or cooling a building needs over a period. Each degree day roughly corresponds to a certain amount of energy consumption, so utilities and energy managers use them to:

• Compare energy usage between months or years, adjusting for weather.
• Estimate heating fuel needs for the winter.
• Size HVAC systems based on historical degree day totals.
• Track plant growth in agriculture (growing degree days).

The concept dates back to the 1920s and remains a standard in the energy industry because it’s simple, consistent, and works well enough for most applications.

Choosing the Right Base Temperature

The base temperature is the outdoor temperature at which no heating or cooling is needed. It varies by application:

• 65°F (18°C) – the standard for residential and commercial building energy in the US. It assumes that internal heat gains (people, appliances) keep the building comfortable at this outdoor temperature.
• 60°F (15.5°C) – sometimes used for buildings with high internal heat gains or better insulation.
• 50°F (10°C) – common for agricultural growing degree days, where plant development halts below this threshold.
• Other bases – you can choose any base that fits your specific process, such as 55°F for wine grape growth or 40°F for some cool-season crops.

If you’re comparing your energy use to a published benchmark, use the same base they used.

Common Mistakes and How to Avoid Them

Even though the formula is simple, errors happen:

• Using the wrong base temperature. Always check which base is standard for your industry or region.
• Forgetting the max(0, …) part. Negative degree days are meaningless; you must zero them out.
• Mixing units. If you use °C, the base should be in °C and the result is in °C·day. Don’t mix Fahrenheit and Celsius.
• Using the daily mean incorrectly. The standard method is (high + low) / 2, not the average of 24 hourly readings. Hourly averages are more accurate but rarely necessary.
• Rounding too early. Round only at the end of a monthly or seasonal total, not each day, to avoid cumulative error.

Limitations of the Degree Day Method

Degree days are a rough approximation. They ignore:

• Solar gain and wind – a sunny, windy day affects heating needs more than the temperature alone suggests.
• Humidity – affects how hot or cold it feels and how much dehumidification is needed.
• Building characteristics – insulation, airtightness, and internal heat gains all change the actual balance point.
• Thermal lag – buildings take time to respond to temperature changes, so a cold morning after a warm day may not require as much heating as the formula suggests.

For precise energy modeling, use hourly weather data and a building simulation tool. But for most practical purposes, degree days are good enough.

Frequently Asked Questions

What is the difference between heating and cooling degree days?

Heating degree days (HDD) measure how cold the day was relative to the base, so they correlate with heating demand. Cooling degree days (CDD) measure how warm the day was relative to the base, correlating with cooling demand. You calculate them with the same mean temperature but subtract in opposite directions, and you always take the positive value.

Can I calculate degree days for a month or a year?

Yes. Add up the daily degree days over the period. For example, if you have 20 HDD on Monday and 15 HDD on Tuesday, the two-day total is 35 HDD. Monthly and annual totals are commonly used for energy analysis and climate comparisons.

What is a growing degree day?

A growing degree day (GDD) uses a base temperature that represents the threshold below which a plant stops developing. For example, for corn, the base is often 50°F. You calculate GDD = max(0, T_mean − 50). The sum of GDD over the season helps predict when a crop will mature.

Why is 65°F used as the base for energy calculations?

It’s a historical standard based on the idea that most buildings need no heating or cooling when the outdoor temperature is 65°F. This was determined from early 20th-century studies of typical building heat gains. It’s still used because it provides a consistent benchmark, even though individual buildings may have different balance points.

Are degree days the same as “degree days� in weather forecasts?

Yes. Weather services and energy companies often report degree days as a measure of how much heating or cooling was needed on a given day. You’ll see them in monthly utility bills or climate data. The formula is the same, but the base temperature may vary by source.

How do I convert degree days from Fahrenheit to Celsius?

Since degree days are a temperature difference times days, you can convert the temperature difference directly: 1°F·day = 5/9 °C·day. So multiply the Fahrenheit degree days by 5/9 to get Celsius degree days (or divide by 1.8).

Related

• Growing Degree Days Calculator (GDD)
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• Average Annual Rainfall: Calculate It Step by Step

in t_high | High temperature | °F | 75 | -130 | 160
in t_low | Low temperature | °F | 55 | -130 | 160
in t_base | Base temperature | °F | 65 | -130 | 160
out hdd | Heating degree days | °F·day | max(0, (t_base - ((t_high + t_low) / 2))) | 1
out cdd | Cooling degree days | °F·day | max(0, ((t_high + t_low) / 2) - t_base) | 1
out mean | Daily mean temp | °F | (t_high + t_low) / 2 | 1