Growing Degree Days Calculator
Calculate growing degree days (GDD, heat units) from daily maximum and minimum temperatures and a base temperature — simple average or the corn 86/50 method — and the days left to reach a crop or insect GDD target.
How it works
GDD = max(0, (T_max + T_min) ÷ 2 − T_base) · modified (86/50) method: T_max and T_min are first limited to the range T_base … T_upper (corn: 10–30 °C, 50–86 °F) · season total = Σ daily GDD · 1 °C·d = 1.8 °F·d
Plants and insects develop at a rate that depends on temperature, and below a base temperature development stops. Growing degree days (heat units) add up how far each day’s mean temperature is above that base, so a crop needs roughly the same GDD to reach a stage whatever the calendar says. Agronomists use them to predict corn and soybean maturity, harvest dates and pest emergence. For corn the modified method caps the maximum at 30 °C (86 °F), because growth does not speed up above it, and treats lows below the base as the base. Totals in °F are 1.8 times those in °C.
Worked example
A day with a maximum of 28 °C and a minimum of 14 °C, base 10 °C, simple method: GDD = (28 + 14) ÷ 2 − 10 = 11.0 °C·d. Thirty such days add 330 °C·d, and a 1,400 °C·d corn hybrid would need 128 days at this rate.
Sources: Iowa State University Extension — Growing degree days for corn · Wikipedia — Growing degree-day
Frequently asked questions
What are growing degree days?
A measure of accumulated heat. Each day contributes its mean temperature minus a base temperature (never less than zero), and the running total predicts when crops or insects reach each growth stage.
What base temperature should I use?
10 °C (50 °F) for corn, soybean, sorghum and many insect models; 4.4–5 °C (40–41 °F) for alfalfa and peas; 0 °C (32 °F) for wheat, barley and oats.
What is the 86/50 method for corn?
Before averaging, the daily maximum is capped at 86 °F (30 °C) and the minimum is raised to 50 °F (10 °C) if lower. It reflects that corn growth does not increase above 86 °F or continue below 50 °F.
How many GDD does corn need?
Typically 2,400–2,800 °F·d (about 1,330–1,550 °C·d) from planting to black layer, depending on the hybrid’s relative maturity — check the seed company’s rating.
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This calculator is for educational purposes. Double-check important results before acting on them.