Calculate growing degree days (GDD) from daily high and low temperatures, base temperature, and crop type to track heat-unit accumulation.
Growing Degree Days Formula
The simple average method sums a day's high and low temperature, averages them, and subtracts the base temperature:
GDD = (Tmax + Tmin) / 2 - Tbase
The modified method first caps the high at an upper cutoff and floors the low at the base temperature, then averages:
GDD = (min(Tmax, Tcut) + max(Tmin, Tbase)) / 2 - Tbase
A season total multiplies the average daily GDD by the number of days, and days to a target divide the target by the average daily GDD:
GDD_total = GDD_daily * Days Days = GDD_target / GDD_daily
- GDD = growing degree days accumulated (heat units)
- Tmax = daily high temperature
- Tmin = daily low temperature
- Tbase = base temperature below which growth stops
- Tcut = upper cutoff temperature above which growth does not increase
- Days = number of days in the period
- GDD_target = total heat units a crop or stage needs
Any negative daily result is set to zero, since cold days do not add heat units. The daily mode returns one day's GDD. The season mode uses an average daily high and low to project a total over many days. The target mode reverses that logic to estimate how many days of similar weather are needed to reach a heat-unit goal such as maturity. The base temperature and upper cutoff both depend on the crop, so the crop preset sets them for you while Custom lets you type your own.
Base and Upper Cutoff Temperatures by Crop
Use these common thresholds when you set the calculator to a crop. Values are in Fahrenheit and are widely used starting points, not fixed rules.
| Crop | Base temp (F) | Upper cutoff (F) |
|---|---|---|
| Corn / sweet corn | 50 | 86 |
| Soybean | 50 | 86 |
| Wheat | 40 | 86 |
| Barley / oats | 40 | 86 |
| Sorghum | 50 | 100 |
| Cotton | 60 | 95 |
| Grapes | 50 | 90 |
| Tomato | 50 | 95 |
| Peanut | 56 | 95 |
Growth stages line up with cumulative heat units better than with calendar days. The table below shows typical corn milestones from planting using a base of 50F and the modified method, so you can read a season total against where the crop should be.
| Corn stage | Cumulative GDD from planting |
|---|---|
| Emergence (VE) | 100 to 125 |
| 6 leaf (V6) | about 475 |
| Tasseling (VT) | about 1135 |
| Silking (R1) | about 1400 |
| Maturity, black layer (R6) | 2500 to 2800 |
Example Problems
Example 1. A corn field has a daily high of 88F and a low of 60F with a base temperature of 50F using the simple method. The average is (88 + 60) / 2 = 74F. Subtracting the base gives 74 - 50 = 24 GDD for that day.
Example 2. The same field averages 24 GDD per day, and a hybrid needs 2700 GDD to reach black layer. Days to maturity are 2700 / 24 = 112.5, which rounds up to 113 days of similar weather.
Frequently Asked Questions
What is a growing degree day? A growing degree day is a single heat unit that measures how much warmth a crop received in one day above the temperature at which it starts to grow. Adding these units across the season tracks development more reliably than counting calendar days, because plants respond to accumulated heat.
When should I use the modified method with a cutoff? Use the modified method when the crop stops gaining development above a certain temperature, which is common for corn and other warm-season crops capped near 86F. On days when the high exceeds the cutoff, the extra heat is not counted, so the modified method reports fewer units than the simple average on hot days.
Which base temperature should I choose? Match the base to the crop. Warm-season crops such as corn, soybean, and sorghum use 50F, while cool-season small grains such as wheat, barley, and oats use 40F. If you know your local extension figure for a specific variety, enter it under the Custom preset.
