Growing degree days calculator
Paste the daily highs and lows from your weather records. The calculator works out the growing degree days of each day and the total, so you can track crop development, predict insect emergence or time sowing and spraying.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Paste the daily maximum temperatures, then the minimums for the same days.
- Choose °C or °F and enter the base temperature for your crop or pest.
- Choose the simple average or the modified method with an upper cutoff.
- Read the total, the average per day and each day’s value.
Simple average: GDD = max(0, (Tmax + Tmin) ÷ 2 − Tbase)
Modified: first Tmax and Tmin are clamped between the base and the upper cutoff, then GDD = (Tmax + Tmin) ÷ 2 − Tbase. A day never adds less than 0 or more than the cutoff minus the base.
Worked example
Five days with highs 24, 27, 19, 22, 31 °C and lows 12, 15, 7, 13, 18 °C, base 10 °C:
| Day |
Mean |
GDD |
| 1 |
18 |
8 |
| 2 |
21 |
11 |
| 3 |
13 |
3 |
| 4 |
17.5 |
7.5 |
| 5 |
24.5 |
14.5 |
Total 44 °C·d, 8.8 a day. With the modified method and a 30 °C cutoff, day 3’s low becomes 10 (4.5 GDD) and day 5’s high becomes 30 (14 GDD), so the total is 45 °C·d.
Tips
- Start counting from a fixed date, such as sowing or 1 March, and keep adding as the season goes.
- Use the published method as well as the base: simple and modified totals differ, especially in cool springs and hot summers.
- Daily highs and lows from a nearby weather station are usually good enough; a sensor in the field is better.
Frequently asked questions
What are growing degree days?
A measure of heat accumulated over time. Plants and insects develop faster when it is warmer, and only above a base temperature. Each day adds its mean temperature minus the base, or nothing if the mean is below the base. Adding up the days predicts stages such as emergence, flowering or maturity better than counting calendar days.
Which base temperature should I use?
The one published for your crop or pest. Common bases are 10 °C (50 °F) for corn, soybean and many insects, 4.4 °C (40 °F) or 0 °C for wheat, barley and peas, and 7 °C (45 °F) for some fruit pests. Development models quote their GDD targets for a specific base, so use the same one.
What is the modified 86/50 method?
The standard for corn in the United States: before averaging, daily highs above 86 °F are set to 86 and lows below 50 °F are set to 50, because corn hardly develops faster above 86 °F. In Celsius the cutoffs are 30 and 10 °C. For the default week it gives 45 instead of 44 degree days.
Can I convert GDD between °C and °F?
Multiply °C degree days by 1.8 to get °F degree days, provided the base was converted too (10 °C = 50 °F). The calculator uses the unit only as a label: enter all temperatures and the base in the same unit.