Growing Degree Days (GDD) Calculator

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