Growing Degree Units Calculator
Maize needs 2,700 units to maturity whether that takes a hot 100 days or a cool 130.
The formula
GDU = (max + min) / 2 - base, floored at zero
Plants keep time by heat, not by date
A crop reaches a growth stage after a certain accumulation of warmth, not after a certain number of days. Growing degree units count that accumulation: each day contributes its mean temperature above a base below which the crop does not develop. Maize needs roughly 2,700 units from planting to maturity whether that takes a hot 100 days or a cool 130.
The 50/86 clamp
Maize and soybean use a method that first clamps the day's maximum at 86 °F and its minimum at 50. Development neither stops below 50 nor accelerates above 86, so counting the extremes would overstate both hot days and cold nights. A day running 45 to 95 °F contributes 18 units, not the 20 an unclamped average would give.
Which base
The base is a property of the crop. Wheat and barley grow down to freezing and use 32 °F; cotton needs 60. Comparing accumulations between crops is meaningless unless the base is the same, which is why the figure is always quoted with it.
Rules of thumb, and the field
Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.