GDU Calculator (growing degree units for corn, 86/50 °F method)

Understanding the GDU calculator — concept and principle

This calculator adds up growing degree units (GDU), also called growing degree days, from a list of daily high and low temperatures. Corn growers and agronomists use GDU totals to track crop development, predict silking and maturity, and compare hybrids rated by their heat-unit requirement.

Each day adds its own heat units; the hottest days are capped at 86 °F, so day 6 adds 29, not 32.5.

How it is calculated

Daily GDU = (high + low) ÷ 2 − base temperature, never below 0

Modified (corn) method: cap the high at the upper limit and raise the low to the base before averaging

For corn the base is 50 °F (10 °C) and the upper limit 86 °F (30 °C).
The total is the sum over all days entered; °C-based units × 1.8 = °F-based units.

The principle

Plants develop faster when it is warmer, roughly in proportion to the heat above a base temperature below which development stops. Adding up the daily heat above the base gives a "thermal clock" that predicts growth stages better than calendar days. The modified method reflects corn's biology: development does not speed up above about 86 °F, and a cold night does not make the crop go backwards, so the high is capped and the low is raised to the base.

Worked example

Default inputs: °F, modified method, base 50, upper limit 86, seven days: 84/62, 90/70, 78/55, 72/48, 88/66, 93/72, 80/58.

Day 1: (84 + 62) ÷ 2 − 50 = 23.
Day 2: high capped at 86 → (86 + 70) ÷ 2 − 50 = 28. Day 3: 16.5.
Day 4: low raised to 50 → (72 + 50) ÷ 2 − 50 = 11. Day 5 (cap): 26.
Day 6 (cap): (86 + 72) ÷ 2 − 50 = 29, the warmest day. Day 7: 19.
Total = 152.5 GDU, average 21.8 per day, or 84.7 in °C units.

How to read the result

Compare the running total with your hybrid's GDU rating to maturity, which seed companies publish. A hot week in midsummer may add 20–30 °F units per day, while cool spring days add little. Because °C and °F units differ by a factor of 1.8, always check which scale a rating uses.

Common mistakes

Mixing °F and °C ratings.
Using the simple average for corn ratings that assume the modified method.
Letting a day count as negative.
Starting the count before planting or emergence when the rating starts at planting.

Keep in mind: GDU predictions are estimates. Soil moisture, stress, day length and hybrid differences also affect development; follow local agronomic advice.

Frequently asked questions

Are GDU and GDD the same?

Yes, growing degree units and growing degree days are two names for the same measure.

Why cap the high at 86 °F?

Corn development does not increase further above about that temperature.

How do I convert a °C total to °F?

Multiply by 1.8.

Sources: R. L. Nielsen, "What Exactly Do You Mean by 'Growing Degree Day'?", Purdue University Agronomy Dept. (2001)

Checked 9 Nov 2026. Nielsen R.L. (Bob), “What Exactly Do You Mean by ‘Growing Degree Day’?”, Purdue University Agronomy Dept., Chat ’n Chew Café, 23 April 2001 (modified GDD for corn: highs above 86 °F set to 86 °F, lows below 50 °F set to 50 °F, GDD = (high + low)/2 − 50).

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