Track heat unit accumulation automatically using your location and live weather data. Enter your crop and planting date — the calculator fetches actual daily temperatures and fills in GDDs every time you return. Multiple plantings, manual overrides, and global coverage.
This calculator uses the USDA Modified method:
GDD = ((min(High, Ceiling) + max(Low, Base)) ÷ 2) − Base
This averages the adjusted daily high and low before subtracting the base temperature,
rather than using the high alone. It accounts for nighttime temperatures — crops accumulate
heat 24 hours a day, not just during the warmest part of the afternoon. The ceiling cap
(86°F for corn) prevents overstating development on very hot days, since growth rate
plateaus above that threshold. This is the standard method used by
Iowa State, Purdue, Kansas State, and WSU Extension for corn and most row crops.
Air temperature vs. soil temperature: GDDs are calculated from air temperature measured at 2 metres above the ground — the standard for all weather stations and agronomic models. However, early-season crop development (germination, emergence, and the first vegetative stages) is driven primarily by soil temperature at seed depth (typically 2 inches / 5 cm), which can differ significantly from air temperature. In spring, dark soils in direct sun can warm 5–15°F above air temperature during the day, meaning the crop may develop faster than air-temp GDDs suggest. Conversely, on cold clear nights soil at seed depth retains heat longer than the air above it. The practical result: GDD-calculated stages often lag visible field stages in early spring, especially in dark irrigated soils like those in the Columbia Basin. The manual stage override (✏️) lets you correct for this by anchoring the projection to what you actually observe in the field.
Automatic GDD accumulation from planting date · weather-powered · persists across visits · worldwide coverage
A growing degree day (GDD), also called a heat unit, measures heat accumulation above a crop's minimum growth temperature. Formula: GDD = ((daily high + daily low) ÷ 2) − base temperature. Negative values are set to zero. GDDs accumulate from planting through harvest to predict development timing.
Corn uses a base temperature of 50°F (10°C) and a ceiling of 86°F (30°C). Daily highs above 86°F are capped at 86°F and daily lows below 50°F are raised to 50°F before averaging. This is the USDA standard modified GDD method. A 100-day corn hybrid typically requires approximately 2,700 GDD from planting to black layer.
Corn typically silks (VT/R1) at 1,135–1,400 GDD (base 50°F) from planting depending on hybrid maturity. A 100-day hybrid reaches black layer at approximately 2,700 GDD; a 115-day hybrid needs 3,100–3,200 GDD. These are averages — always use your seed company's specific GDD rating for your hybrid.
Winter and spring wheat most commonly use a base of 32°F (0°C) with no upper ceiling for vegetative stages. Some models use 40°F (4.4°C). Wheat requires approximately 2,000–2,500 GDD (base 32°F) from planting to harvest. Use the Custom Crop option in this calculator to enter any base and ceiling temperature.
GDD (growing degree days) and GDU (growing degree units) are the same calculation with different regional names. GDU is common in the Upper Midwest and Canada; GDD is used in the western US and internationally. Both use: average daily temperature − base temperature, with negative values set to zero.
Yes. GDD is widely used for pest management timing. Corn rootworm egg hatch begins near 684 GDD (base 50°F). Western bean cutworm peak flight occurs near 780–900 GDD. Soybean aphid thresholds are often linked to crop development stage rather than calendar date. GDD-based timing is more reliable than calendar date because it accounts for actual growing conditions.
Soybeans use base 50°F (10°C) and ceiling 86°F (30°C), the same as corn. Full-season soybeans (Group II–IV) typically require 2,000–2,700 GDD from planting to physiological maturity depending on maturity group and latitude. Earlier maturity groups require fewer GDD.
GDD models are typically accurate within 3–7 days for major milestones like silking and black layer under normal conditions. Accuracy decreases with drought, flooding, disease, or frost. Calibrate GDD thresholds to your specific hybrid — seed company GDD ratings for your product are always more accurate than generic published values. Use the Crop Yield Calculator alongside GDD tracking to project end-of-season revenue.
15 crops plus custom: Corn/Maize/Mealie, Soybeans/Soya Beans, Winter Wheat (HRW/SRW), Spring Wheat (CWRS/HRS), Durum/Pasta Wheat, Barley/Malting Barley, Oats, Grain Sorghum/Milo/Jowar, Canola/Oilseed Rape/Colza, Sunflower, Dry Beans/Navy/Pinto/Haricot, Lentils/Masoor/Pulses, Field Peas/Marrowfat, Potatoes (Solanum tuberosum), Sugar Beet/Fodder Beet, and a Custom crop with user-defined base and ceiling temperatures. International terminology is built in for North America, UK, EU, South Asia, and Southern Africa.
Yes. Click the ✏ Edit button on any planting card to open the form pre-filled with that planting's crop, date, label, variety, and GDDs to maturity. Your GDD log is fully preserved when you save. If you change the crop or base temperature, all log entries are automatically recalculated. If you move the planting date forward, entries before the new date are trimmed.
