Growing Degree Days (GDD) Calculator
Track heat unit accumulation to predict flowering, grain filling and the likely harvest date of your crop.
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Please note: This calculator provides a mathematical estimate based on the values entered. Recommended agricultural application rates can vary by crop, soil, product formulation, location and official agricultural guidance. Check the product label and local agricultural recommendations before application.
Crops grow by heat, not by the calendar. Enter the average maximum and minimum temperatures, your crop's base temperature and the days since sowing, and see the growing degree days so far and how many days remain to the stage you are waiting for.
About the Growing Degree Days (GDD) Calculator
The same wheat variety can flower in 75 days in a warm year and 90 days in a cool one. What stays nearly constant is the amount of heat it needs. Growing degree days (GDD), also called heat units or thermal time, measure that heat.
Each day, a crop gains heat equal to how much the day's average temperature is above its base temperature – the temperature below which it hardly develops. A day averaging 16.5 °C gives a wheat crop (base 5 °C) 11.5 GDD. Add the days up, and you can see how far the crop has progressed toward a stage such as flowering or maturity.
This calculator uses the widely used modified average method:
- the maximum is capped at an upper limit (often 30 °C), because extra heat does not speed growth;
- the minimum is raised to the base temperature if it falls below it;
- negative values count as zero.
With average temperatures for the period, it gives a quick estimate. For best accuracy, calculate each day from daily readings and add them up.
How to use the Growing Degree Days (GDD) Calculator
- 1 Enter the average daily maximum and minimum temperatures since sowing (from a nearby weather station or agro-met advisory).
- 2 Enter the base temperature for your crop and the upper limit.
- 3 Enter the days since sowing and the GDD your variety needs to reach the stage you are interested in.
- 4 Press Calculate to see the heat units so far and the days remaining.
The formula we use
GDD per day = (min(Tmax, Tupper) + max(Tmin, Tbase)) ÷ 2 − Tbase (0 if negative)
Accumulated GDD = Sum of daily GDD
Days remaining = (Target GDD − Accumulated GDD) ÷ GDD per dayWorked example
Wheat sown 60 days ago. Average maximum 24 °C, minimum 9 °C, base 5 °C. Suppose your variety needs 1,600 GDD to maturity.
- GDD per day = (24 + 9) ÷ 2 − 5 = 11.5.
- GDD so far = 11.5 × 60 = 690 (43% of the way).
- Still needed = 910 GDD, about 80 more days at the same temperatures. As spring warms up, it will be fewer.
Things to keep in mind
- Use the same base temperature as the source of your target GDD – values based on 0 °C and 5 °C are very different.
- Temperatures change through the season. Recalculate every couple of weeks with updated averages.
- Stress changes timing. Drought and heat stress can hasten maturity regardless of GDD.
Frequently asked questions
What is the base temperature?
The temperature below which a crop develops very little. Cool-season crops like wheat and mustard have a base near 5 °C; warm-season crops like rice and maize about 10 °C.
How many GDD does wheat need?
It depends on the variety and the base temperature used. Use the figure published for your variety by your state agricultural university or ICAR institute.
Why cap the maximum temperature?
Above an upper limit, crop development does not speed up further and may even slow, so counting that extra heat would overestimate progress.
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