What is a growing degree day (GDD)?

A growing degree day (GDD), also called a heat unit or growing degree unit (GDU), measures heat accumulation above a crop's minimum growth temperature on a given day. Formula: GDD = ((daily high + daily low) / 2) minus the base temperature. If the result is negative it is set to zero. GDDs accumulate from planting through harvest to predict crop development timing.

How does the automatic GDD tracking work?

When you set your location, the GDD Tracker fetches historical daily high and low temperatures from Open-Meteo, a free global weather API using ECMWF ERA5 reanalysis data. It calculates GDD for each day from your planting date through yesterday and adds them to your log automatically. Each time you return to the page, it detects the gap since your last visit and fills in all the missing days — you never need to enter temperatures manually.

Does the GDD tracker work outside the United States?

Yes. Open-Meteo covers the entire globe at approximately 9km grid resolution. The tracker supports international crop terminology: corn is listed as Corn/Maize/Mealie, canola as Canola/Oilseed Rape/Rapeseed/Colza, soybeans as Soybeans/Soya Beans, and wheat by regional variety names including CWRS, HRS, HRW, and SRW. Celsius and Fahrenheit are both supported. Type any city, region, or postal code worldwide to set your location.

What base temperature does wheat use for GDD?

Winter and spring wheat use a base temperature of 32 degrees F (0 degrees C) with no upper ceiling temperature. This matches the most widely used agronomic standard. Wheat typically requires 2,000 to 2,500 GDD (base 32 F) from planting to harvest maturity depending on variety and region.

What is the difference between GDD and GDU (growing degree units)?

GDD and GDU are the same calculation with different regional names. GDU is commonly used in the Upper Midwest United States and Canada. GDD is used in the western US, UK, Europe, and internationally. Both formulas are identical: average daily temperature minus base temperature, with negative values set to zero.

Can I track multiple crops or fields at the same time?

Yes. Click Add Planting to add as many tracked plantings as you need. Each planting has its own crop type, planting date, variety name, GDD log, and milestone tracking. All plantings are saved in your browser and restored automatically on every visit. Click any planting card to switch the active view.

Can I override automatic temperatures with my own field readings?

Yes. Click Add or Override Day to enter your own high and low temperatures for any date. If that date already has an auto-filled entry it will be replaced. This is useful when your field runs warmer or cooler than the weather grid — for example near an irrigation canal, on a south-facing slope, or in a frost pocket. Manual entries are marked with a pencil icon in the log.

How accurate is the weather data used for GDD calculations?

Open-Meteo uses ECMWF ERA5 reanalysis data at approximately 9km grid resolution. It is typically accurate to within 1 to 3 degrees F for most agricultural locations. Accuracy decreases in complex terrain, near water bodies, in frost pockets, and during radiative frost events where local temperatures can differ from the grid estimate by 5 to 10 degrees F or more. For critical decisions, verify with an on-farm sensor or nearby official weather station.

What crops does the GDD tracker support?

The tracker supports 15 crops plus custom: corn (maize, mealie), soybeans (soya beans), winter wheat (HRW, SRW), spring wheat (CWRS, HRS), durum wheat (pasta wheat), barley (malting barley), oats, grain sorghum (milo, jowar), canola (oilseed rape, rapeseed, colza), sunflower, dry beans (navy, pinto, haricot), lentils (masoor, pulses), field peas (marrowfat), potatoes (Solanum tuberosum), sugar beet (fodder beet), and a fully custom crop with user-defined base and ceiling temperatures.

Growing Degree Day (GDD) Tracker

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.

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GDD Calculator

Automatic GDD accumulation from planting date · weather-powered · persists across visits · worldwide coverage

Tracked Plantings

Crop & Temperature Unit

Crop

Temperature Unit

Base: 50°F Ceiling: 86°F Method: Modified (USDA) Season total: 0 GDD

Location — for automatic weather data

📍 No location set — GDDs will be entered manually

Add Daily Temperature

Date

Daily High (°F)

Daily Low (°F)

Tip — add days in sequence from planting date. Date auto-advances after each entry.

Season Summary —

Crop: Planted: Base: Method: Season Total:

Season Summary

Total GDD

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Avg GDD / Day

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Projected Days to Maturity

Development Milestones

Daily Log

Date	High (°F)	Low (°F)	Avg	GDD	Season Total	Stage	Source

Weather Data & Location Accuracy — Temperature data is sourced from the Open-Meteo API using ECMWF ERA5 reanalysis, a gridded atmospheric model with global coverage at approximately 9 km (5.6 mi) grid resolution, updated daily. This is not data from a specific weather station — it represents a modeled estimate interpolated to the nearest grid point to your coordinates. Actual field temperatures commonly differ from this estimate by 1–5°F (0.5–3°C) and can differ by more due to local conditions including elevation changes, slope aspect, proximity to rivers or irrigation canals, urban heat islands, frost pockets, or overhead canopy. In complex terrain or during radiative frost events, discrepancies can exceed 10°F. Weather data typically lags 1–5 days; the most recent days in your log may not yet be populated. For critical agronomic or spray timing decisions, verify with an on-farm temperature sensor or a nearby official weather station record.

GDD Milestone Accuracy — Development stage milestones shown in this tool are general agronomic averages based on published research and are not calibrated to a specific hybrid or variety. Actual crop development can vary by 5–15% from these averages depending on the hybrid GDD rating, planting depth, seed-to-soil contact, soil temperature (which lags air temperature by several days in spring), soil moisture, plant population, and regional adaptation. A hybrid or variety rated at 2,650 GDD by its seed company may reach physiological maturity anywhere from 2,500 to 2,800 accumulated GDD depending on the growing season. Always cross-reference with your seed company’s hybrid-specific GDD ratings and your own field scouting records. This tool is designed for planning and season monitoring only — it is not a substitute for direct field observation.

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.