GDU Calculator — Growing Degree Units

Quick worked example: corn in mid-summer

Let's say you are growing corn (base 10°C, max 30°C) and today reached 28°C with a low of 17°C.

A solid mid-summer day for corn. Over a 90-day growing season, accumulating 12–15 GDU per day adds up to well over 1,000 GDU — more than enough for most corn varieties to reach maturity.

It's mid-July and your cornfield has been experiencing a stretch of hot days. You want to know how many growing degree units have accumulated today to track whether your crop is on pace for a timely harvest.

The daily maximum of 32°C is clamped down to 30°C (the crop maximum), and the minimum of 18°C stays above the 10°C base. The result: 15 GDU for this day. Corn typically needs around 100–120 GDU to emerge, and about 2,500–3,000 GDU over the entire season to reach maturity — so every day counts!

You are growing tomatoes in a greenhouse where temperatures are more controlled. The day started cool at 12°C, peaked at 26°C, and your tomato variety has a base temperature of 10°C and a maximum growing temperature of 30°C.

Both temperatures fall within the crop limits, so no clamping occurs: (26 + 12) / 2 - 10 = 9 GDU. Tracking these daily values helps you predict when your tomatoes will reach flowering and fruit-set stages.

It is early spring and you have just planted wheat. The base temperature for wheat is 0°C, and its maximum growing temperature is 30°C. Today reached a high of 15°C but dropped to −2°C overnight.

The overnight low of −2°C is floored to 0°C (the base temperature), since temperatures below the base do not contribute to growth. The GDU is (15 + 0) / 2 − 0 = 7.5. A cool day but still some growth — wheat is quite cold-tolerant.

• Know your crop's base temperature

  Different crops have different base temperatures. Corn sits around 10°C (50°F), wheat around 0°C (32°F), and soybeans around 10°C (50°F). Using the wrong base will throw off your entire season's GDU tracking.

• Use reliable temperature data

  For best accuracy, use temperatures recorded at or near your field. Weather station data from a nearby airport can differ from your actual farm conditions by several degrees, especially in hilly terrain.

• Track GDU daily, not weekly

  Averaging temperatures over a week can hide cold snaps or heat waves. The daily calculation captures the real growing conditions much more accurately. Record each day's result and sum them up to track progress toward your crop's maturity target.

• Combine with other crop management tools

  GDU tells you about heat, but plants also need the right spacing, nutrients, and soil. Use our Plant Spacing Calculator to optimize your garden layout, and the Fertilizer Calculator to match nutrient applications to growth stages.

Step-by-step breakdown

• 1Clamp the daily maximum: $T_{\text{eff,max}} = \min(T_{\max}, T_{\mathrm{crop}})$. If the actual high exceeds the crop's maximum growing temperature, use the crop maximum instead. Extra heat above that point does not help the plant grow.
• 2Floor the daily minimum: $T_{\text{eff,min}} = \max(T_{\min}, T_{\mathrm{base}})$. If the overnight low drops below the base temperature, use the base instead. Temperatures below the base do not contribute to growth.
• 3Average the effective temperatures and subtract the base: $\text{GDU} = \frac{T_{\text{eff,max}} + T_{\text{eff,min}}}{2} - T_{\mathrm{base}}$.

Why clamp?

Without clamping, a 40°C day would produce a very high GDU — but in reality, corn stops benefiting from heat above about 30°C. Similarly, a night at −5°C would subtract from the daily average, but growth simply stops below the base — it doesn't go backward. The clamping adjustment makes GDU a realistic measure of useful heat.

GDU vs GDD: same thing, different name

You may see Growing Degree Units (GDU) and Growing Degree Days (GDD) used interchangeably. They mean the same thing — the accumulated heat above a crop's base temperature. Some sources use “units” to emphasize that the value is calculated per day, while “days” refers to the cumulative total. In practice, both terms refer to the same metric.

Typical GDU requirements for common crops

These are rough estimates. Actual requirements vary by variety, region, and management practices. Always check seed package or local extension service recommendations.

Why GDU matters for practical farming

Calendar dates are an unreliable guide for crop management because temperatures vary so much from year to year and region to region. A crop planted on May 1 might reach flowering in late June one year but mid-July the next, depending on the weather. GDU smooths out this variability by measuring what actually drives plant growth: heat. This allows farmers to schedule irrigation, fertilizer application, pest management, and harvest based on the plant's true developmental stage rather than an arbitrary date.

For advanced crop modeling, also explore our Daily Light Integral (DLI) Calculator to track how much light your plants receive — another key growth factor alongside heat accumulation.