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 on calendar days; they develop on the warmth they receive. GDD gives farmers, gardeners and agronomists a way to predict germination, flowering, pest emergence and harvest that a date on a calendar cannot.

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)

Degree days carry a unit, and this is where people go wrong. A degree day computed from Fahrenheit temperatures is 1.8 times smaller than one computed from Celsius, because a Fahrenheit degree is smaller. The two are not interchangeable, and almost nobody writes the unit down. American extension services publish in °F-days at base 50°F; European and Canadian sources publish in °C-days at base 10°C. When you read that corn needs 2,700 GDD, that is the Fahrenheit figure. The same corn needs 1,500 in Celsius degree days. This calculator reports in whichever unit you pick and labels it, and it scales the crop milestones to match.

GDD Milestones for Common Crops

Wheat and other small grains use a lower base, 40°F or 4.4°C, and run roughly 1,800 to 2,200 °F-days from seeding to harvest.

Multi-Day Calculation This calculator repeats one daily high and low across the number of days you give it, so what comes back is the accumulation under a steady weather pattern. A real season needs real daily records, and a warm fortnight followed by a cold one does not average out the way the arithmetic here implies. Use it to ask what a forecast run of weather is worth, not to reconstruct a season after the fact.

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 °F-days

Note that the two examples describe almost the same weather and give 110 and 195. That is not an error, it is the unit. 110 °C-days × 1.8 = 198, and the small remainder is the gap between 28°C/14°C and 82°F/57°F.