Growing Degree Units (GDU) Calculator

What Is a Growing Degree Unit?

A growing degree unit (GDU), also called a growing degree day (GDD), is a measure of how much heat a crop has received on a given day. Farmers and agronomists add up GDUs from planting through the season to predict when a crop will reach a stage such as emergence, flowering, or maturity. This calculator finds the daily GDU value from a day's high temperature, low temperature, and a crop's base temperature.

Counting calendar days alone does not work well, because a cool day and a warm day push a plant forward by different amounts. GDU fixes this by turning temperature into a running score of accumulated heat.

How to Calculate Growing Degree Units

Every crop has a base temperature: the point below which it stops developing. Below that temperature, the plant's growth processes slow to almost nothing, so those hours add no GDU. The base temperature for corn is 50°F (10°C), and soybean models commonly use the same value.

Many crops also have an upper limit. For corn it is about 86°F (30°C); above it, extra heat does not speed up development further and conditions become stressful for the plant. This calculator uses the modified GDU method (sometimes called the "corn" or Barger method), which sets daily highs above 86°F to 86°F and daily lows below 50°F to 50°F before averaging. The calculator applies the same adjustment with whatever base temperature is entered. The formula is:

$\text{GDU} = \frac{\text{clamped } T_{\text{max}} + \text{clamped } T_{\text{min}}}{2} - T_{\text{base}}$

Where each temperature is first adjusted like this:

• If a temperature is below the base temperature, it is raised to the base temperature.
• If a temperature is above 86°F, it is lowered to 86°F.
• Otherwise it is left unchanged.

If the result of the formula is negative, the calculator reports 0, since a crop cannot lose accumulated heat.

Why the temperatures are adjusted first

The adjustment step matters more than it looks. Without it, a very cold night could pull the daily average below the base temperature and produce a misleading negative or near-zero score, even though the plant grew normally during the warm daytime hours. Capping the high temperature stops unusually hot days from being credited with more growth than a plant can actually use. Because both adjustments happen before the average is taken, this calculator's result can differ from a plain, uncapped average of the day's high and low.

Worked Examples

Example 1: A hot day with a capped high

• Maximum temperature: 92°F
• Minimum temperature: 68°F
• Base temperature: 50°F

The maximum is above 86°F, so it is capped: 92°F becomes 86°F. The minimum, 68°F, is already above the base temperature, so it is unchanged.

$\text{Average} = \frac{86 + 68}{2} = 77°F$

$\text{GDU} = 77 - 50 = 27.00$

Example 2: A cool night with a raised low

• Maximum temperature: 65°F
• Minimum temperature: 40°F
• Base temperature: 50°F

The maximum, 65°F, is between the base temperature and 86°F, so it stays the same. The minimum, 40°F, is below the base temperature, so it is raised to 50°F.

$\text{Average} = \frac{65 + 50}{2} = 57.5°F$

$\text{GDU} = 57.5 - 50 = 7.50$

Example 3: Tracking a running total

GDU values are added together across a season. The table below tracks five days with a base temperature of 50°F.

On Day 3, the minimum of 45°F is below the base temperature and is raised to 50°F before averaging, giving (70 + 50) / 2 − 50 = 10.00 GDU. Day 4 works the same way: the minimum of 40°F is raised to 50°F, giving (65 + 50) / 2 − 50 = 7.50 GDU. After five days, the crop has accumulated 77.50 GDU.

Base Temperatures by Crop

Base temperature depends on the crop's origin and biology. A crop bred for a cool climate tolerates lower temperatures than one bred for the tropics.

These values come from agricultural extension guidance. Kansas State University lists lower temperature limits of 50°F for corn, 32°F for winter wheat, 46°F for sorghum and 60°F for upland cotton. North Dakota State University uses 32°F (0°C) as the base for wheat and the corn limits of 50°F and 86°F for soybeans. Cotton heat units are often called "DD-60s" because they use a 60°F base temperature. Seed suppliers sometimes publish a different base temperature for a specific variety, and that value should be used instead when available.

What Growing Degree Units Are Used For

Farmers use accumulated GDU totals for several practical decisions:

• Predicting growth stages. Seed companies publish approximate GDU totals for milestones such as emergence, tasseling, or maturity, so a grower can estimate when a stage will arrive instead of guessing from the calendar.
• Choosing planting dates. Comparing a variety's total GDU requirement against the typical GDU accumulated by the first fall frost shows whether there is enough of a safety margin to plant later or earlier.
• Timing pest scouting. Many insect pests develop on their own heat-driven schedule. Extension services publish GDU thresholds for when to start scouting for specific pests.
• Scheduling irrigation and harvest. Because water-sensitive growth stages such as flowering occur at known GDU ranges, growers can anticipate them instead of reacting after the fact.

History

The idea of adding up daily temperatures to predict plant development is much older than modern corn science. The corn method used by many calculators today was first tested by E. C. Gilmore and J. S. Rogers in 1958, who called it "Effective Degrees". In 1969, G. L. Barger proposed the same method, under the name "Modified Growing Degree Days", as the standard heat-unit formula for the US National Oceanic and Atmospheric Administration.

Frequently Asked Questions

Is a growing degree unit the same as a growing degree day? Yes. GDU and GDD refer to the same measurement. Different regions and organizations favor one term or the other, but the calculation is identical.

Why does this calculator cap the high temperature at 86°F? For corn, extra heat above about 86°F does not speed up development, and the plant may experience heat stress instead. Capping the high temperature keeps a single very hot day from overstating that day's contribution to plant growth.

Why is my result different from a simple average of the high and low? This calculator adjusts each temperature toward the base temperature and the 86°F cap before averaging. A cold night below the base temperature is raised to the base temperature, and a hot day above 86°F is lowered to 86°F. A plain average of the raw high and low will not match this calculator whenever a temperature falls outside that range.

What base temperature should I use? Common values are 50°F for corn and soybeans, 46°F for sorghum, 32°F for wheat, and 60°F for cotton. Use a seed supplier's value instead when one is given for a particular variety.

How do I track GDU across a whole season? Calculate the daily GDU for each day starting at planting or emergence, then add each day's value to a running total. Compare the total against published GDU requirements for the growth stage of interest.

Can GDU predict harvest exactly? No. It narrows the likely window considerably compared with counting calendar days, but drought, flooding, nutrient stress, and variety differences can all shift the actual date.

References

1. North Dakota Agricultural Weather Network (NDAWN), North Dakota State University. Corn Growing Degree Days. NDAWN corn GDD help — base temperature for corn of 50°F (10°C); upper limit of 86°F (30°C), above which conditions are stressful.
2. Purdue University Agronomy. Heat Unit Concepts Related to Corn Development. Purdue corn heat units — the Modified GDD method: highs above 86°F set to 86, lows below 50°F set to 50; Gilmore and Rogers (1958) and Barger (1969).
3. Kansas State University Extension. Understanding Growing Degree Days (MF3740). K-State MF3740 — lower temperature limits: corn 50°F, winter wheat 32°F, sorghum 46°F, upland cotton 60°F.
4. North Dakota Agricultural Weather Network (NDAWN). Wheat Growth Stage Prediction Using Growing Degree Days. NDAWN wheat GDD help — 32°F (0°C) lower threshold for wheat GDD.
5. North Dakota Agricultural Weather Network (NDAWN). Growing Degree Day Model for North Dakota Soybean. NDAWN soybean GDD help — the standard corn thresholds of 50°F and 86°F used for the soybean model.
6. University of Georgia Cooperative Extension. Cotton Growth Monitoring and PGR Management (C 1244). UGA cotton heat units — cotton heat-unit model with a 60°F base temperature (DD-60s).