Introduction
The GDU Calculator shows how much heat your crop has soaked up since planting day. GDU stands for Growing Degree Unit. It is also called GDD, or Growing Degree Day. Plants do not grow by the calendar. They grow by heat. So counting heat tells you more about your crop than counting days.
The math is simple. Add the day's high temperature and low temperature, divide by two, then subtract a base temperature. For corn, the base is 50°F: daily highs above 86°F are counted as 86, and daily lows below 50°F are counted as 50.1 Add up each day, and you get your season total.
Pick your field location, your planting date, your crop, and your hybrid or relative maturity. The tool then shows your GDUs to date, how that compares to the 30-year normal, and what the next 7 and 14 days should add. It also gives you estimated dates for key growth stages like V4 to V7, silking, and black layer.
Use it to plan scouting trips, time your nitrogen or fungicide passes, guess at pollination timing, and see if your hybrid will hit maturity before the first fall freeze. Works for corn, soybeans, wheat, sorghum, and cotton, in °F or °C, for fields in the contiguous United States (the lower 48 states).
How to use our GDU Calculator
Enter your field location, your planting date, and your crop or hybrid. The calculator shows total growing degree units (GDUs) so far, how that compares to normal, a 7-day forecast, what the next 14 days should add, charts, and the dates your crop should hit each growth stage.
Search ZIP code or city: Type at least two letters or numbers, then pick your town from the list. This fills in your field's latitude and longitude. The tool covers fields in the contiguous United States (the lower 48 states).
Latitude: Type the degrees north of your field if you know them. Use a value between 24 and 50.
Longitude: Type the degrees east of your field. Use a negative number for west, between -125 and -66.5.
View Map Selector: Click this to open a map. Click a spot or a city dot, or use the arrow keys, then click "Use This Location."
Planting date: Pick the day you planted. All GDUs add up from this date forward.
Base temperature method: Choose the base temperature used in the math. Corn growers usually use base 50°F with the 86/50°F cap. Click the info button to see how each one works.
Temperature unit: Pick °F or °C. Every temperature, threshold, and GDU total changes to match.
Crop type: Choose corn, soybeans, wheat, sorghum, or cotton. This sets the default base method, growth stages, and maturity range.
Lookup method: Pick "Seed guide rating" to enter the GDU ratings your seed company publishes, or "Relative maturity" to type a maturity number and let the tool estimate them.
GDUs to silking and to maturity: Type the two ratings from your seed guide, as GDUs from planting.
Relative Maturity: Type the CRM days or maturity group for your seed. The tool then estimates the GDUs needed for each stage.
Overlay layers: Check the boxes for the lines you want on the chart, like growth stage thresholds, your custom target, and historical or freeze dates.
Custom GDU target: Type or slide to any GDU number you care about. The chart and stage table show the date you should reach it.
Season view: Click "Season to Date" for observed weather plus the 7-day forecast, or "Season Outlook" to see the rest of the season using 30-year normals. Past daily highs and lows come from the PRISM daily temperature grid, which covers the continental United States from 1981 to the present.4 The 30-year normal is the average of the 1991 to 2020 PRISM seasons over the same dates, leaving out any season with a missing day. The forecast comes from the National Weather Service grid forecast for your point, which covers the next seven days.5 Days that are not in the forecast yet use the normal.
Timeline controls: Use the green slider to shift your planting date and the brown sliders to zoom the chart to a smaller date range.
Chart inspector: Slide to any day to read its high, low, daily GDU, and total GDU in words.
Calculate and Reset: Click Calculate to update your results. Click Reset to return every input to its starting value.
What Are Growing Degree Units (GDUs)?
Growing Degree Units, also called GDUs or Growing Degree Days (GDD), measure heat. Crops do not grow by the calendar. They grow by warmth. A warm week pushes corn ahead fast. A cool week slows it down. GDUs count that heat so you can track how far along your crop really is.
How GDUs Are Counted
Each day, you take the high temperature and the low temperature, average them, and subtract a base temperature. The base is the temperature below which the crop stops growing. If the answer is less than zero, the day counts as zero.
Corn uses a special rule. Highs above 86°F are counted as 86°F, and lows below 50°F are counted as 50°F. Corn does not grow faster on a 100°F day, and it barely grows on a 45°F night. Then you subtract 50. This "Modified Growing Degree Days" method is the one most commonly used in the U.S. for tracking corn development.1
Base Temperatures by Crop
- Corn: base 50°F, with the 86/50 cap
- Soybeans: base 50°F
- Wheat: base 32°F
- Sorghum: base 50°F
- Cotton: base 60°F (often called DD60s)
Why Farmers Track GDUs
GDUs add up from planting day. As the total grows, the crop moves through its stages. Each stage needs a set amount of heat. For example, one widely used corn GDD tool gives a 112-day hybrid a default rating of 2,691 GDUs from planting to black layer, which is full maturity.3
Knowing the running total helps you plan:
- Spray and side-dress timing. Many products are labeled by growth stage, not date.
- Scouting. You know what stage to look for before you walk the field.
- Pollination window. Silking is the most heat- and water-sensitive time for corn.
- Harvest planning. You can guess when grain will dry down and when combines are needed.
- Frost risk. If the crop is behind, an early fall freeze can cut yield and test weight.
Relative Maturity and Hybrid GDU Ratings
Relative Maturity (RM or CRM) is the "day" number on a corn seed bag, like 108-day or 114-day. It is not a real count of days. It is a rating that compares hybrids. Higher numbers need more heat and take longer to finish. A hybrid's maturity is more reliably described by the GDUs it needs from planting to black layer, and most seed companies publish those ratings.3 If your seed company rates a hybrid from emergence instead of from planting, add about 115 GDUs.3 Soybeans use Maturity Groups instead, from 00 in the far north to 8 in the Deep South.
Normals, Forecasts, and Comparisons
A 30-year normal is the long-term average heat for your spot on the map. A 15-year rolling average leans on more recent seasons. Comparing your season to normal tells you if the crop is ahead or behind. Being 150 GDUs behind normal in August can mean a later harvest and wetter grain.
What Changes Your GDU Total
- Location. South and low ground pile up heat faster than north and high ground.
- Planting date. Planting late means fewer warm days left in the season.
- Weather swings. Cool, cloudy stretches stall the count.
- Night temperatures. Warm nights add GDUs, but they can also burn stored sugars and trim yield.
Things to Keep in Mind
GDUs only track heat. They do not know about dry soil, hail, disease, compaction, or short nitrogen. A stressed field can hit its GDU target and still look behind. Late-planted corn also tends to need fewer GDUs to reach black layer than the seed bag says: in Purdue research, hybrids planted after about May 1 matured in about 6.8 fewer GDUs for each day of delay.3 Use GDUs as a strong guide, then check the field to be sure.