What this calculator does
Growing degree days measure heat accumulation rather than elapsed time, because crop development tracks temperature far better than it tracks the calendar. The daily figure is the mean of the maximum and minimum, less a base temperature below which the crop does not develop.
Base temperature is crop-specific and choosing it wrongly makes the whole accumulation meaningless. Corn and most warm-season crops use 10°C, while wheat and other cool-season crops use around 0 to 5°C. A GDU total is only comparable against another calculated with the same base.
The formula
The daily mean temperature is the average of the maximum and minimum. The base temperature is subtracted from it to give the daily growing degree units, and that is multiplied by the number of days to give the accumulation. This is the simple averaging method; refinements exist that cap the maximum at an upper threshold, which this does not apply.
| Term | Meaning |
|---|---|
| GDU | Growing degree unit, one degree-day of accumulated heat above the base. |
| Base temperature | The threshold below which development effectively stops. 10°C for corn, lower for cool-season crops. |
| Daily mean | The average of the daily maximum and minimum, used as the day representative temperature. |
| Upper threshold | A cap on the maximum used in some refinements, since development does not keep accelerating indefinitely. Not applied here. |
The inputs explained
| Field | What to enter |
|---|---|
| Average daily high temperature (°C) | Daily maximum temperature in Celsius. |
| Average daily low temperature (°C) | Daily minimum temperature in Celsius. |
| Base temperature (crop threshold) (°C) | The crop base temperature. 10°C is standard for corn and most warm-season crops. |
| Number of days | Number of days at this temperature pattern, for the accumulated total. |
When to use it
Predicting crop development stages
Stages such as tasselling and maturity are specified in accumulated GDU for a given hybrid, so tracking the accumulation predicts them better than counting days.
Comparing seasons
Two seasons with the same number of days can differ substantially in heat accumulation, and GDU is what makes them comparable.
Timing pest management
Insect development is also temperature-driven, and degree-day models with pest-specific base temperatures predict emergence and life stages for spray timing.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does maximum temperature change GDU accumulation?
A range of daily maximum temperatures with the minimum and base held fixed over 30 days.
| Daily maximum | Accumulated GDU | Daily GDU | Average daily mean |
|---|---|---|---|
| 20 °C | 225.0 GDU | 7.50 GDU/day | 17.50 °C |
| 25 °C | 300.0 GDU | 10.00 GDU/day | 20.00 °C |
| 30 °C | 375.0 GDU | 12.50 GDU/day | 22.50 °C |
| 35 °C | 450.0 GDU | 15.00 GDU/day | 25.00 °C |
Questions
What base temperature should I use?
10°C for corn, soybeans and most warm-season crops. Wheat, barley and other cool-season crops use somewhere between 0 and 5°C depending on the model. Insects have their own base temperatures, often quite different. Using the wrong base makes the accumulation incomparable with published targets.
What happens if the mean is below the base temperature?
The day contributes zero rather than a negative amount, since development stops rather than reversing. This calculator applies the simple formula directly, so check the result if you are entering temperatures near or below the base.
Why not just count days?
Because development responds to temperature, not to the calendar. A cool spring delays a crop well beyond what the date suggests, and two plantings a fortnight apart can reach the same stage within days of each other if the second ran into warmer weather. GDU captures that and days do not.
What is the upper threshold refinement?
A cap on the maximum temperature used in the calculation, commonly 30°C for corn, reflecting that development does not keep accelerating above that and eventually slows. The simple method used here has no cap, so it overstates accumulation during genuinely hot spells.
For light accumulation rather than heat, see the daily light integral calculator. For field work rates, see the acres per hour calculator.