How are growing degree days calculated?

Average the day's maximum and minimum temperature, subtract the base temperature for the crop, and count the result if it is positive. A day averaging 18 degrees against a base of 10 contributes 8 growing degree days. A day averaging below the base contributes nothing rather than a negative number, because cold does not undo growth already made, and treating it as negative is the most common way this calculation goes wrong.

What base temperature should I use?

It depends entirely on the crop, because the base is the temperature below which that particular plant does not grow. Maize is commonly calculated on 10 degrees and cool season grasses and cereals on a lower base, but the right figure comes from the agronomy for your crop and variety rather than from a general rule. No default is supplied here for that reason: the base is the input that decides everything else.

Why use growing degree days instead of the calendar?

Because plants respond to accumulated warmth rather than to elapsed days. A cold spring and a warm one produce the same growth stage at quite different dates but at similar degree day totals, which is why degree days predict flowering, maturity and pest generations better than the calendar does. It is also why a target expressed in degree days travels between seasons and regions in a way that a date never does.

Growing Degree Days Calculator NZ

Plants respond to accumulated warmth rather than to elapsed days, which is why a cold spring and a warm one reach the same growth stage weeks apart on the calendar and at almost the same degree day total. This works out the growing degree days a day contributes, what has accumulated over a run of days, and how far off a target you are at the rate you have been going. The arithmetic is simple: average the day's maximum and minimum, subtract the base temperature for your crop, and count what is left. The part that is easy to get wrong is the floor. A day that averages below the base contributes nothing, not a negative number, because a cold snap does not undo growth the crop has already made. Allowing negative days is the single most common error in a degree day calculation and it will quietly understate the accumulation for the whole season. The base temperature is the input that decides everything else, and it belongs to the crop rather than to the method: it is the temperature below which that particular plant does not grow. No default is supplied here, because a base borrowed from the wrong crop makes every figure downstream of it wrong.

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Daily maximum, average over the period

Daily minimum, average over the period

Base temperature for the crop

Days counted so far

Target for the growth stage

GDD

292.5

growing degree days accumulated

A day6.5 GDD

Still to go1,107.5 GDD

Days at this rate171 days

Share of target

A day averaging below the base contributes nothing, not a negative number. Cold does not undo growth the crop has already made.

No base temperature or growth stage target is supplied here. Both belong to the crop and the variety rather than to the method, and a base borrowed from the wrong crop makes every figure below it wrong. Take them from the agronomy for what you are actually growing.

How it works

The mean temperature for a day is the maximum plus the minimum divided by two. Subtracting the base gives the degree days that day contributed, floored at zero. Multiplying by the days gives the accumulation, which is an approximation when you use period averages rather than day by day figures, and a good one as long as no individual day fell below the base. The days remaining figure divides what is left of the target by the current daily rate, which assumes the weather continues as it has been, and that assumption is the weakest part of any forward estimate here.

Why the zero floor matters more than it looks

Because negative days accumulate. If a calculation allows a day averaging 8 degrees against a base of 10 to subtract 2, then a fortnight of cool weather removes 28 degree days that the crop never actually gave back, and the error compounds across a season rather than averaging out. Growth stalls when it is cold, it does not reverse. Flooring at zero is not a convenience, it is what makes the total mean what it claims to mean.

Worked example

A run of 45 days averaging 22 degrees by day and 11 by night has a mean of 16.5 degrees. Against a base of 10 for maize, that is 6.5 growing degree days a day and 292.5 accumulated over the period, which is 20.9 percent of a 1,400 degree day target. At the same rate the remaining 1,107.5 degree days would take another 171 days, which is the useful warning in the figure: either the crop needs warmer weather than it has had, or the target will not be reached this season. A run averaging 25 by day and 13 by night would give 9 degree days a day, reaching the same 1,400 target in 156 days from the start rather than 216.

Related calculators

Grain Yield Estimator: what the crop is carrying.
Farm Feed Budget: what the pasture is growing meanwhile.
Grain Moisture Shrink: what happens after harvest.
Celsius to Fahrenheit: if your figures are in Fahrenheit.

Data sources: the growing degree day calculation is a standard method rather than a rate, and it is the same everywhere. Base temperatures and growth stage targets belong to the crop and the variety, so neither is supplied here.