Growing Degree Days Formula

Growing degree days (GDD) measure heat accumulation over a season, calculated as the average of the daily high and low temperatures minus a base temperature. You enter your daily maximum and minimum temperatures (and optionally a base temperature), and the calculator returns the GDD for that day. This value helps predict plant development, pest emergence, and irrigation scheduling.

Formula

The standard GDD formula is:

GDD = (T_max + T_min) / 2 − T_base

where:

• T_max = daily maximum air temperature (in °F or °C)
• T_min = daily minimum air temperature (in °F or °C)
• T_base = the base temperature below which the organism does not grow (in °F or °C)

If the result is negative, it is set to zero—no growth occurs when the average temperature is below the base. This is called the “average method� or “rectangle method.�

Worked Example

Using the default values from the calculator: max = 75°F, min = 55°F, base = 50°F.

1. Average temperature = (75 + 55) / 2 = 65°F
2. Subtract base: 65 − 50 = 15
3. Since 15 > 0, GDD = 15 degree-days (°F-day)

The calculator shows 15.0 °F-day, matching the step-by-step calculation.

Why Base Temperature Matters

The base temperature (T_base) is the threshold below which the organism’s development stops. It varies by species:

• Corn: 50°F (10°C)
• Wheat: 40°F (4.4°C)
• Tomato: 50°F (10°C)
• Many insects: 50°F (10°C) for aphids, 52°F (11°C) for some beetles

Using the wrong base gives misleading GDD totals. Always confirm the base for your specific crop or pest from a reputable agricultural extension source.

How to Accumulate GDD Over a Season

To track development, sum daily GDD values from a start date (e.g., planting or biofix). For example, if you have daily GDDs of 10, 12, 8, and 15 over four days, the accumulated GDD is 10+12+8+15 = 45. Many crops require a specific total GDD to reach a stage:

• Corn silking: ~1400–1500 GDD (base 50°F)
• Wheat heading: ~1200–1500 GDD (base 40°F)
• Tomato fruit set: ~1000–1200 GDD (base 50°F)

These numbers are general; local varieties and conditions vary.

Common Mistakes and Limitations

One frequent error is using Celsius temperatures with a Fahrenheit base, or vice versa. Always keep units consistent—convert if needed. Another mistake is ignoring the “no negative� rule: if the average is below base, GDD is zero, not negative. The average method is simple but can overestimate on days with extreme swings. More advanced methods (e.g., sine curve) are more accurate but require hourly data. GDD also assumes a linear response to temperature, which is not true at very high or low extremes.

Alternatives to the Average Method

If you have hourly temperature data, the sine curve method fits a sine wave between the daily min and max and integrates the area above the base. This gives a more accurate GDD. Another alternative is the modified growing degree day that caps the maximum temperature (e.g., 86°F for corn) to avoid overcounting heat stress. For chill hours (for fruit trees), you use a different calculation with a base of 45°F, but that measures cold, not heat.

Practical Uses in Agriculture and Gardening

GDD helps you:

• Predict when to spray for pests (e.g., codling moth emerges at 250–300 GDD base 50°F)
• Schedule planting and harvesting
• Estimate crop maturity for insurance or marketing
• Plan irrigation based on growth stage

For home gardeners, tracking GDD from last frost can tell you when to plant warm-season crops. Many local extension services publish GDD accumulations for your area.

Frequently Asked Questions

What is the difference between GDD and growing degree units (GDU)?

They are the same thing—GDU is just another name for GDD. Both use the same formula and units (degree-days).

Can I use GDD for indoor plants?

Yes, if you have temperature data. For indoor plants, the base temperature is often lower (e.g., 40°F), and you can use the same formula with your indoor highs and lows.

What if my low temperature is below the base but the average is above?

That’s fine—you still calculate GDD using the average. The formula doesn’t care about individual hours; it only uses the daily average. If the average is above base, you get a positive GDD.

How do I convert GDD from Fahrenheit to Celsius?

You don’t convert the GDD value directly; you recalculate using temperatures in Celsius. The base temperature also changes (e.g., 50°F = 10°C). One degree-day in Celsius is 1.8 times larger than in Fahrenheit, but it’s easier to just use consistent units.

Why do some sources use a maximum temperature cap?

Some crops, like corn, stop growing above a certain temperature (e.g., 86°F). If you use a cap, you replace any max above 86°F with 86°F before averaging. This prevents overestimating growth on hot days.

Can I calculate GDD for a whole week at once?

Yes, but you need daily data. Sum the daily GDDs for each day of the week. Do not just average the weekly high and low—that loses daily variation and undercounts GDD.

Related

• Inches of Rain: How Rainfall Is Measured
• Snow Water Equivalent Calculator (Snow to Rain)
• Growing Degree Days Calculator (GDD)

in tmax | Max temperature | °F | 75 | -130 | 160
in tmin | Min temperature | °F | 55 | -130 | 160
in base | Base temperature | °F | 50 | -40 | 100
out gdd | Growing degree days | °F-day | (max((tmax+tmin)/2 - base, 0)) | 1
out avg | Average temperature | °F | (tmax+tmin)/2 | 1