Growing Degree Days (GDD) Calculator
Calculate growing degree days (GDD = ((Tmax + Tmin)/2) - base) for crops from daily high and low temps.
Returns cumulative GDD for planting timing.
Growing Degree Days (GDD) — also called heat units — measure the accumulation of heat available for plant and insect development over a growing season. Plants and insects do not develop based on calendar days alone; they develop based on the amount of warmth they receive. GDD gives farmers, gardeners, and agronomists a precise way to predict biological events like germination, flowering, pest emergence, and harvest time.
The Formula GDD per day = ((T_max + T_min) / 2) − T_base
Where:
- T_max = daily maximum temperature
- T_min = daily minimum temperature
- T_base = base (threshold) temperature below which no development occurs
If the result is negative (mean temperature below the base), GDD = 0 for that day. Development does not go backward.
Standard Caps Most agricultural models apply a ceiling to T_max of 30°C (86°F) and floor T_min at the base temperature. This prevents overestimation: at very high temperatures, some crops slow down or stop development (heat stress). Capping T_max at 30°C prevents those days from inflating the GDD count.
Base Temperature by Crop Different crops and pests have different biological thresholds:
- 10°C / 50°F (base 10): Used for corn, soybeans, sorghum, the most common agricultural base
- 4.4°C / 40°F (base 40): Used for wheat, barley, oats, cool-season grains that grow in colder conditions
- 7.2°C / 45°F (base 45): Used for grapes and some stone fruits
- 10°C / 50°F: Also used for tracking many insect pests (aphids, codling moth)
GDD Milestones for Common Crops
- Corn: needs approximately 2,700 GDD (base 10°C) to reach full maturity from planting
- Tomatoes: need about 1,000 GDD (base 10°C) from transplant to first harvest
- Wheat: needs 1,800–2,200 GDD (base 4.4°C) from seeding to harvest
- Sweet corn: approximately 700–900 GDD (base 10°C) from planting to harvest
- Soybeans: 2,400–3,000 GDD (base 10°C) depending on variety
Multi-Day Calculation This calculator applies the same daily high and low temperature repeated over the number of days you specify. For a full season, you would need daily weather data. The result gives you the GDD accumulated if every day has the same conditions — useful for estimating development under a given weather pattern.
Worked Example — Celsius Daily high: 28°C, Daily low: 14°C, Base: 10°C, for 10 days: Mean = (28 + 14) / 2 = 21°C GDD/day = 21 − 10 = 11 GDD 10 days × 11 = 110 GDD accumulated
Worked Example — Fahrenheit Daily high: 82°F, Daily low: 57°F, Base: 50°F, for 10 days: Mean = (82 + 57) / 2 = 69.5°F GDD/day = 69.5 − 50 = 19.5 GDD 10 days × 19.5 = 195 GDD accumulated