Growing Degree Days (GDD)
Enter daily max/min temperature, crop base temperature and days to compute daily mean, daily GDD and accumulated GDD over the period, for predicting crop phenology and growth stages.
Input Data
Results
At a glance:Growing degree days (GDD), also called effective accumulated temperature, sum up day by day the 'effective heat above the crop base temperature' to quantify how temperature drives development. The development rate of poikilothermic organisms and plants is mainly set by temperature: growth advances only when temperature exceeds a 'base temperature' (Tbase, the threshold for growth to start). The standard method first takes the daily mean Tavg = (Tmax + Tmin) ÷ 2, then daily GDD = max(0, Tavg − Tbase) — if the mean is below base, that day contributes no effective heat and is recorded as 0; summing daily GDD across the growth period gives accumulated GDD. Each cultivar has a fairly fixed accumulated-GDD requirement from sowing to heading, flowering and maturity, so GDD predicts phenology more accurately than mere 'calendar days', and is used to schedule farm work, forecast pest generations and choose suitable varieties. In practice an upper temperature cap is often set (development stops accelerating above it), and summing with real daily temperatures is most accurate; this calculator, for quick estimation, uses an isothermal approximation from your entered mean — daily GDD × days — to gauge the heat magnitude over a period.
Formula
Daily mean: Tavg = (Tmax + Tmin) ÷ 2.
Daily GDD: GDD_day = max(0, Tavg − base temperature Tbase).
Period accumulated (isothermal approx.): total = daily GDD × days.
$$T_{avg} = \frac{T_{max} + T_{min}}{2}$$$$GDD_{day} = \max(0,\; T_{avg} - T_{base})$$$$GDD_{total} = GDD_{day} \times n$$How to Use
- Enter the (or representative) daily max and min temperature and the crop base temperature Tbase.
- Enter the number of days approximated by this mean temperature.
- The tool returns daily mean, daily GDD and accumulated GDD; when mean is below base, daily GDD is 0.
Approximate base temperature Tbase for common crops
| Crop | Base Tbase (°C) | Note |
|---|---|---|
| Wheat / barley (cool-season) | 0–4 | Cold-tolerant, low base |
| Maize | 10 | Common warm-season base |
| Soybean | 10 | Similar to maize |
| Tomato / cotton | 10–15 | Warm-loving, higher base |
Base temperature varies by crop and study; a wrong base systematically over/underestimates GDD, so use local/cultivar recommendations.
Case Studies
Accumulated heat of maize over 60 days
Representative weather: Tmax = 30°C, Tmin = 18°C, maize base 10°C, about 60 days.
Tavg = (30 + 18) ÷ 2 = 24°C; daily GDD = 24 − 10 = 14 °C·day.
60-day accumulated GDD = 14 × 60 = 840 °C·day; if the cultivar needs ~800 GDD to head, it heads around this period.
Cold period contributes no heat
Early spring: Tmax = 8°C, Tmin = 4°C, maize base 10°C.
Tavg = (8 + 4) ÷ 2 = 6°C, below base 10°C.
Daily GDD = max(0, 6 − 10) = 0 — maize barely develops, accumulated GDD 0, showing early sowing of warm-season crops just waits without growing.
FAQ
Why use accumulated GDD instead of days to predict phenology?
Development is temperature-driven — warm days grow fast, cold days slow. The same calendar days give very different development under different weather. Accumulated GDD sums the daily effective heat above base, measuring progress by 'heat' rather than 'time', automatically reflecting warm/cold differences. So the same cultivar's heading and maturity correspond to a fairly fixed accumulated GDD, not a fixed number of days.
How to choose the base temperature Tbase?
Base temperature is the minimum for growth and varies by crop: cool-season wheat/barley ~0–4°C, warm-season maize/soybean ~10°C, warm-loving tomato/cotton ~10–15°C. A wrong base systematically over/underestimates GDD (too low counts invalid low temperatures). Use the base recommended by local agriculture departments or the cultivar specification.
Is daily GDD negative when mean is below base?
No. Daily GDD is defined as max(0, Tavg − Tbase), taking the larger of 'mean minus base' and 0. When mean is below base the difference is negative but GDD is recorded as 0, meaning that day adds no development (cold does not reverse growth). This calculator has the lower-bound built in.
Does this tool give exact daily accumulation?
This tool uses an isothermal approximation from your entered mean (daily GDD × days) for a quick magnitude estimate. The most accurate way is to enter real max/min daily, compute daily GDD and sum — because actual temperature changes every day. For rigorous phenology forecasts, accumulate daily meteorological data; this tool suits estimation and teaching.
Should I set an upper temperature cap?
Many crops stop accelerating or are even inhibited above a certain upper temperature; standard GDD often caps temperatures above the upper limit (e.g. maize ~30°C) to avoid overestimating heat on hot days. This calculator uses the basic method (only a base lower bound, no upper cap) for generality and simplicity; if your crop has a clear upper temperature, truncate the mean or max by the cap before entering.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.