Growing Degree Days Calculator

Growing degree days (GDD), also called heat units, are a measure of accumulated thermal energy above a crop's minimum growth temperature (base temperature). GDD is calculated daily as: GDD = ((T_max + T_min) / 2) - T_base, where T_max and T_min are daily maximum and minimum temperatures and T_base is the base temperature below which crop development stops. Negative daily GDD values are set to zero. GDD accumulation predicts crop development stages, pest emergence and harvest timing.

Base temperatures vary by crop: corn (maize) uses 50 deg F (10 deg C); soybeans use 50 deg F; wheat uses 32 deg F (0 deg C); cotton uses 60 deg F (15.5 deg C); tomatoes use 50 deg F. Pest management also uses GDD: corn rootworm egg hatch occurs at approximately 684 GDD base 50 deg F; black cutworm adult flight peaks near 300 GDD base 50 deg F. These values are published by USDA and land-grant university extension services.

The simple average method used here (average of daily max and min minus base temperature) is the most common approach and is specified by the NOAA NWS and the Iowa State University Extension. More sophisticated methods include the single-sine and double-triangle methods, which better account for diurnal temperature variation and upper threshold temperatures. The simple average method is accurate within about 5 to 10% of the sine method for most practical crop management purposes.

Many crops have an upper temperature threshold (T_upper) above which development slows or stops. For corn, USDA NRCS uses T_upper = 86 deg F (30 deg C): if T_max exceeds 86 deg F, it is capped at 86 deg F; if T_min exceeds 86 deg F, it is capped at 86 deg F. This calculator uses only the base temperature threshold; if you need the capped method for corn, apply T_max = min(T_max, 86) and T_min = min(T_min, 86) before entering values.

Corn requires approximately 2,100 to 2,800 GDD base 50 deg F to reach physiological maturity (black layer formation), depending on the maturity rating of the hybrid. Early-season hybrids in northern US may need 2,100 to 2,300 GDD; full-season hybrids in southern US need 2,500 to 2,800 GDD. The crop's seed tag or variety trial data provides the specific GDD requirement. Source: USDA NRCS and University of Illinois Extension.