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. Corn and soybeans commonly use a base temperature of 50°F (10°C).

Most crops also have a practical upper limit. Above about 86°F (30°C), extra heat does not speed up development further and can even stress the plant. This calculator uses the modified GDU method (sometimes called the "corn" or Barger method), which caps the daily high at 86°F and raises the daily low up to the base temperature before averaging. 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 are commonly used defaults from agricultural extension guidance. 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 summing temperature to predict plant development dates back to the 1730s, when French scientist René Réaumur proposed that the total of daily temperatures above a threshold could be used to forecast when a plant would reach a given stage. Researchers refined the concept over the following two centuries by introducing crop-specific base temperatures. The corn heat-unit method used by many modern calculators, including the modified method with an upper temperature cap, traces back to a 1958 paper by E. C. Gilmore and J. S. Rogers that measured corn maturity using accumulated heat units.

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? Above roughly 86°F, most crops stop gaining developmental benefit from extra heat and 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? Use 50°F for corn, soybeans, and sorghum; 32°F for wheat; and 60°F for cotton, unless a seed supplier specifies a different value 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. Gilmore, E. C., and J. S. Rogers. "Heat Units as a Method of Measuring Maturity in Corn." Agronomy Journal, vol. 50, no. 10, 1958, pp. 611–615.
2. McMaster, G. S., and W. W. Wilhelm. "Growing Degree-Days: One Equation, Two Interpretations." Agricultural and Forest Meteorology, vol. 87, no. 4, 1997, pp. 291–300.
3. Baskerville, G. L., and P. Emin. "Rapid Estimation of Heat Accumulation from Maximum and Minimum Temperatures." Ecology, vol. 50, no. 3, 1969, pp. 514–517.