Growing Degree Days Calculator
Calculate daily growing degree days (GDD) to track crop development and predict harvest timing. Enter daily high, low, and crop-specific base temperatures to get your result.
About this calculator
Growing degree days (GDD) measure heat accumulation above a threshold temperature that a crop needs to complete a biological stage. The formula is: GDD = max(0, ((maxTemp + minTemp) / 2) − baseTemp). The average of the daily maximum and minimum temperatures gives the mean daily temperature. Subtracting the base temperature — the minimum below which the crop makes no developmental progress — yields the heat units available that day. The max(0, …) function ensures negative values (cold days) contribute zero rather than subtracting from the total. GDD values are summed across the growing season; when cumulative GDD reaches a crop-specific threshold, a physiological event such as germination, silking, or maturity is expected. Common base temperatures are 50°F for corn and 40°F for wheat.
How to use
Suppose corn is your crop (base temperature = 50°F), today's high was 88°F, and today's low was 62°F. Step 1 — mean temperature: (88 + 62) / 2 = 75°F. Step 2 — subtract base: 75 − 50 = 25 GDD. Step 3 — apply max(0, …): since 25 > 0, today contributes 25 GDD. If corn needs roughly 2,700 cumulative GDD to reach maturity, and you average 20 GDD/day, you would expect maturity in about 135 days.
Frequently asked questions
What base temperature should I use for different crops in the growing degree days formula?
Base temperature varies by crop species and reflects the biological minimum for growth. Corn and sorghum typically use 50°F (10°C), while cool-season crops like wheat and barley use 32–40°F (0–4°C). Soybeans commonly use 50°F, and cotton uses 60°F. Using the wrong base temperature will shift your cumulative GDD totals and produce inaccurate predictions for planting or harvest windows. Consult extension service tables or seed company data sheets for the precise base temperature recommended for a specific variety.
How are growing degree days used to predict pest emergence and crop harvest timing?
Accumulated GDD act as a biological clock that is more reliable than calendar dates because they reflect actual thermal conditions experienced by the organism. Agronomists use GDD thresholds to predict when insects like corn rootworm will hatch or when fungal disease pressure peaks, allowing timely pesticide applications. Crop consultants track GDD from planting to predict silking, grain fill, and physiological maturity dates. This information helps farmers schedule harvest equipment, negotiate contracts, and plan drying costs.
Why does the growing degree days formula use max(0, result) instead of allowing negative values?
Biological development essentially halts below the base temperature, so cold days should not subtract from accumulated heat units — they simply contribute nothing. The max(0, …) function enforces this by flooring any negative daily value at zero. Without this constraint, a very cold night combined with a warm afternoon could produce a negative average that reduces your running GDD total, implying the crop 'lost' development — which has no biological basis. Some advanced GDD methods also cap the maximum temperature to avoid over-counting heat on extremely hot days.