Calculate daily growing degree days (GDD), a constant-weather total estimate, or days to a heat-unit target from daily high and low temperatures. Choose a custom model or the corn 86/50°F high-cutoff model.
Growing Degree Days Formula
The simple average method sums a day’s high and low temperature, averages them, and subtracts the base temperature:
GDD = max(0, (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 = max(0, (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. Exact observed accumulation requires calculating and summing each day separately; applying cutoffs to a season’s average high and low can give a different answer. The corn preset uses 50°F/86°F; other crop models require locally supported custom thresholds.
Base and Upper Cutoff Temperatures by Model
Use thresholds specified by a crop-development model or local extension guidance. Only the sourced corn high-cutoff preset is supplied; unsupported universal cutoffs for other crops have been removed. Fahrenheit degree-days convert to Celsius degree-days by multiplying by 5/9; no 32-degree offset applies to heat-unit differences.
| Crop | Base temp (F) | Upper cutoff (F) |
|---|---|---|
| Corn high-cutoff model | 50 (10°C) | 86 (30°C), applied to daily high |
| Other crops or local models | Use a supported custom base | Use the cutoff specified by that model, if any |
Growth stages can relate to cumulative heat units. The table below gives approximate reference values for a mid-season corn hybrid, not universal maturity thresholds; actual requirements vary by hybrid and conditions.
| Corn stage | Cumulative GDD from planting |
|---|---|
| Emergence (VE) | about 125 |
| 6 leaf (V6) | about 475 |
| Tasseling (VT) | about 1150 |
| Silking (R1) | about 1400 |
| Maturity, black layer (R6) | about 2700 |
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. In a hypothetical constant-weather projection, a field accumulates 24°F·day per day and the remaining target is 2700°F·day. 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 both the base and the method to a crop-specific model and local extension guidance. The supplied corn high-cutoff model uses 50°F (10°C) as its base and caps the daily high at 86°F (30°C). For other crops or varieties, choose a custom model and enter supported thresholds.
