Growing Degree Days Calculator
Crops count heat, not days.
Work out Growing Degree Days. Crops count heat, not days. Counts the thing everyone forgets to count.
Comma separated, one per day, in °C.
Growing degree days
63
7 days at 9 a day · base 10 °C · simple
Growing degree days count heat rather than time, which is why the same crop matures in 90 days one year and 110 the next while needing the same GDD in both. It is the only reason a planting date and a harvest date can be linked at all. The simple and modified methods diverge substantially in a hot season — often 15% or more — and published GDD targets rarely say which they used. Comparing a modified accumulation against a simple target predicts a harvest that does not arrive. The base temperature is crop-specific: 10 °C for maize, 4.5 °C for wheat, 0 °C for many cool-season grasses. Using the wrong base is a larger error than choosing the wrong method, and it is easier to get wrong because both are just numbers on a chart.
How the Growing Degree Days Calculator works
GDD accumulation by both the simple and modified methods, with progress towards a maturity target and days remaining at the current rate. The two methods diverge substantially in a hot season.
Also known as: how to calculate growing degree days · heat units for maize · gdd to maturity · simple versus modified gdd
Frequently asked questions
How are growing degree days calculated?
Take the mean of the daily maximum and minimum, subtract the base temperature, and accumulate. Days where the mean falls below the base contribute zero, never a negative.
What is the modified method?
It caps the daily maximum at an upper threshold before averaging, because growth does not keep accelerating with heat. Above the threshold most crops slow rather than speed up.
Does it matter which method I use?
Substantially. The two diverge by 15% or more in a hot spell, and published GDD targets rarely say which they used — comparing a modified accumulation against a simple target predicts a harvest that never comes.
What base temperature should I use?
It is crop-specific: 10 °C for maize, 4.5 °C for wheat, 0 °C for many cool-season grasses. Using the wrong base is a larger error than choosing the wrong method.
Why use GDD instead of days?
Because the same crop matures in 90 days one year and 110 the next while needing the same heat. GDD is the only thing that links a planting date to a harvest date across seasons.
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