Calculate growing degree days (GDD) from a daily high, low, and crop base temperature — standard, modified 86/50, and cutoff methods, in °C or °F.
Simple average of the daily high and low, minus the base temperature.
Assumes the same daily high and low each day — a planning estimate.
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Growing Degree Days (GDD) measure accumulated heat that drives plant development. Unlike calendar days, GDD accounts for actual temperature conditions, providing a more accurate way to predict crop maturity and plan agricultural activities.
Growing degree days (GDD), or heat units, measure the heat accumulated above a crop's base temperature — the temperature below which it makes no growth. Each day's GDD is the average of the daily high and low minus the base temperature, and is never below zero. Summed over the season, GDD predicts development stages far better than calendar days. Corn, for example, uses a base of 50°F with the modified 86/50 method (Ohio State AGF-101, UC IPM).
GDD Formula
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The base temperature is the threshold below which a crop makes no measurable growth, so heat below it isn't counted. It's 50°F (10°C) for corn, soybeans, and grapes, 40°F (4.4°C) for wheat, and 45°F (7°C) for apples. Use your crop's published base — degree days are only meaningful relative to it.
The standard method averages the raw daily high and low, then subtracts the base. The modified 86/50 method (used for corn) first caps the high at 86°F and raises the low to 50°F. The cutoff method caps both the high and low at the upper threshold without raising the low. They agree on mild days and diverge only at temperature extremes.
Common base temperatures are corn 50°F (10°C), wheat 40°F (4.4°C), soybeans 50°F (10°C), tomatoes 50°F (10°C), and cotton 60°F (15.6°C). Your local university extension service publishes crop- and region-specific values — the calculator sets these automatically when you pick a crop.
No. When the average of the daily high and low falls at or below the base temperature, that day contributes zero GDD — never a negative value. Cold days simply add nothing to the seasonal total rather than subtracting from it.
No — degree days carry the unit of the temperature scale, so 1 GDD°F equals about 0.56 GDD°C (GDD°F = GDD°C × 1.8). A corn crop needing 1,400 GDD at base 10°C needs 2,520 GDD at base 50°F. Always keep the base temperature and the target in the same unit.