Degree Days: How to Calculate Them

Degree days measure how much the outdoor temperature deviates from a comfortable baseline, helping you estimate heating or cooling energy needs. You enter your daily high and low temperatures plus a base temperature, and the calculator returns heating or cooling degree days for that day.

Formula

The most common method is the average method (also called the mean method). It uses the daily average temperature, which is simply the high plus the low divided by 2:

Daily average temperature = (High + Low) ÷ 2

Then compare that average to a base temperature — the outdoor temperature at which you neither heat nor cool your building. Common bases are 65°F (18°C) for general energy use and 50°F (10°C) for plant growth. The formulas are:

Heating degree days (HDD) = max(0, Base − Daily Average)

Cooling degree days (CDD) = max(0, Daily Average − Base)

The max(0, …) part ensures the result is never negative; if the average is above the base, heating degree days are zero, and vice versa.

Worked Example

Using the calculator defaults: high = 65°F, low = 45°F, base = 65°F.

1. Find the daily average: (65 + 45) ÷ 2 = 110 ÷ 2 = 55°F
2. Heating degree days: max(0, 65 − 55) = 10 HDD
3. Cooling degree days: max(0, 55 − 65) = max(0, −10) = 0 CDD

So that day contributed 10 heating degree days and 0 cooling degree days.

How to Read the Result

A single day’s degree days are small — typically 0 to 40. The real value comes from summing them over a month or season. For example, 30 days of 10 HDD each gives 300 HDD for the month. That total correlates strongly with the amount of fuel or electricity you used for heating, so you can compare one month to another, or one year to the next, even if temperatures varied.

Heating degree days are positive in winter, cooling degree days in summer. If you track both, you can see which season dominates your energy use. Many utility companies use degree days to normalize bills — a colder-than-normal month will have more HDD, explaining higher heating costs.

Common Mistakes

• Using the wrong base temperature. 65°F is standard for buildings, but greenhouses or crop models often use 50°F or even 40°F. The same weather data gives very different degree-day totals with a different base.
• Forgetting the max(0, …) rule. If you simply subtract, you get negative numbers on warm winter days. Those must be counted as zero, not negative, or your totals will be wrong.
• Mixing Fahrenheit and Celsius. Degree days are quoted in °F·day or °C·day. If you use Celsius temperatures with a Fahrenheit base, the numbers are meaningless. Convert everything to one system first.
• Using the day’s high as the average. The average method uses the midpoint between high and low, not just the high. Using only the high overestimates cooling degree days and underestimates heating ones.
• Rounding too early. Round the final total, not each day’s value, to avoid small errors accumulating over a month.

Tips

• Use daily data, not monthly averages. A month with 60°F average could be 30 days of 60°F or 15 days of 40°F and 15 of 80°F. The degree-day totals differ significantly because of the max(0) rule.
• For a season, sum the daily values. Most energy analysts use October–April for heating and May–September for cooling in the northern hemisphere, but your local climate may differ.
• If you only have monthly average temperature, you can approximate degree days, but the result is less accurate. Daily data is always better.
• For plant growth, use the base temperature specific to the plant species — many crops use 50°F, but some cool-season plants use 40°F. Check a reliable agricultural source for your crop.
• Track both HDD and CDD. Even in winter, a few days may contribute CDD if the average exceeds the base; those matter for buildings with internal heat gains.

Troubleshooting

My result is zero even though it was cold. Check that your high and low are entered correctly. If the average is above the base, HDD is zero by definition — that is correct. Also confirm you are looking at the HDD output, not the CDD one.

The numbers seem too large or too small. Verify the units. A day with a 30°F average and 65°F base gives 35 HDD — that is normal for a cold winter day. If you get 350, you may have entered the temperature in Celsius but the base in Fahrenheit, or summed 10 days instead of 1.

My monthly total does not match my energy bill. Degree days measure temperature, not actual fuel use. Your heating system’s efficiency, thermostat settings, insulation, and occupant behavior all affect consumption. Degree days are a comparison tool, not a bill predictor.

I only have the daily mean temperature, not the high and low. You can still compute degree days: enter the same value for both high and low. The average will equal that mean, and the formulas work correctly.

I need degree days in Celsius. Enter temperatures in °C and use a base in °C (commonly 18°C). The result will be in °C·day. Do not mix units.

Frequently Asked Questions

What is a base temperature? The base temperature is the outdoor temperature at which no heating or cooling is needed. For buildings, 65°F (18°C) is standard; for plants, it varies by species, often 40–55°F.

What is the difference between heating and cooling degree days? Heating degree days measure how far the average temperature falls below the base, indicating heating demand. Cooling degree days measure how far it rises above the base, indicating cooling demand. They are mutually exclusive for a single day.

Can I calculate degree days for a week or month directly? You can approximate by using the average of all daily averages, but the result will be less accurate than summing daily values. The max(0) rule makes the sum nonlinear, so direct monthly averages undercount degree days.

Why do some sources use 65°F and others 60°F? The base temperature is a modeling choice. 65°F is the traditional standard for residential energy analysis, but newer building standards or specific climates may use a different base. Always check which base a source uses before comparing numbers.

Are degree days the same as growing degree days? No. Growing degree days (GDD) use a base temperature specific to a plant and often an upper limit above which growth stops. They are calculated the same way but with different bases and sometimes a cap on the daily maximum.

How accurate is the average method? The average method is a good approximation but can be off by a few percent on days with unusual temperature curves. More accurate methods (like the sine-wave method) exist, but for most energy analysis and agricultural planning, the average method is sufficient.

Related

• Growing Degree Days Calculator (GDD)
• Snow to Rain Calculator: Convert Snowfall to Rain
• Average Annual Rainfall: Calculate It Step by Step

in hi | Daily high temp | °F | 65 | -130 | 160
in lo | Daily low temp | °F | 45 | -130 | 160
in base | Base temp | °F | 65 | 0 | 100
out hdd | Heating degree days | °F·day | max(0, base - (hi + lo) / 2) | 1
out cdd | Cooling degree days | °F·day | max(0, (hi + lo) / 2 - base) | 1