A free growing degree days calculator for any crop
Crops develop with heat, not with calendar days: a cool spring slows them and a hot spell speeds them up. Growing degree days (GDD, also called growing degree units or heat units) measure that heat. This growing degree days calculator adds them up from daily maximum and minimum temperatures, using the base temperature of your crop, in °F or °C, and tells you which growth stages have been reached.
For corn it uses the standard 86/50 method, and it carries the stage targets of a 2,700-GDD corn hybrid and cotton DD60 milestones. Paste your own weather-station data or type a few days; nothing leaves your browser.
How to use the GDD calculator
- Choose °F or °C. Switching later converts the numbers you have typed.
- Pick the crop: the base temperature, upper threshold and method fill in. Choose Custom to type your own.
- Paste daily highs and lows, one day per line, with or without a date in front.
- Optionally enter the planting date and a target GDD — for example the maturity rating of your hybrid.
- Read the total, the chart, the stages reached and the days to the next stage; download the daily table as CSV.
Growing Degree Days Calculator features
Two methods
Plain average (days below the base count zero) or the modified method that caps the maximum and raises the minimum to the base, used for corn.
Crop base temperatures
Corn, sorghum, soybean and rice 50 °F; cotton 60 °F; peanut 56 °F; potato and sunflower 45 °F; small grains 40 °F or 32 °F.
Stage targets
V2 to black layer for a 2,700-GDD corn hybrid; emergence to open boll for cotton.
°F and °C
All inputs and stage targets switch between Fahrenheit and Celsius degree days.
Chart and CSV
Cumulative GDD line with stage lines, and a downloadable daily table.
The growing degree days formula
GDD for one day = (Tmax + Tmin) ÷ 2 − Tbase, and a negative result counts as 0. Add the days together from planting to get accumulated GDD.
Corn (86/50 method). Before averaging, any maximum above 86 °F is set to 86 and any minimum below 50 °F is set to 50, because corn hardly grows faster above 86 °F or at all below 50 °F. A day of 75/55 °F gives (75 + 55) ÷ 2 − 50 = 15 GDD. A day of 98/66 °F gives (86 + 66) ÷ 2 − 50 = 26 GDD — not 32, because the 98 is capped.
Celsius. One °C degree day equals 1.8 °F degree days. Corn’s 50/86 °F becomes 10/30 °C, and a 2,700 °F-GDD hybrid needs 1,500 °C-days.
Base temperatures and stage targets
Base temperatures are from the University of Florida IFAS publication AE428; North Dakota’s NDAWN network uses 32 °F for wheat. Corn targets come from the National Corn Handbook (NCH-40) and cotton from University of Georgia Extension. Hybrids rated 2,400–2,800 GDD reach each stage proportionally earlier or later.
| Crop | Base °F | Base °C | Typical targets (°F GDD from planting) |
|---|---|---|---|
| Corn (maize) | 50 (cap 86) | 10 (cap 30) | V6 475 · VT 1,135 · R1 silking 1,400 · black layer 2,700 (2,700-GDD hybrid) |
| Cotton | 60 | 15.6 | Emergence 50 · pinhead square 550 · first bloom 940 · open boll 2,150 · harvest 2,500–2,700 |
| Sorghum, soybean, rice | 50 | 10 | Use the variety’s rating |
| Peanut (groundnut) | 56 | 13.3 | — |
| Potato, sunflower | 45 | 7.2 | — |
| Wheat, barley, oats, canola | 40 (or 32) | 4.4 (or 0) | — |
Tips and limits
- Start counting at planting (or at emergence if your targets are given from emergence) — mixing the two shifts every stage by about 100–120 GDD in corn.
- Use air temperatures from a nearby station at 1.5–2 m height. The daily-average method is an approximation; hourly methods give slightly different totals.
- Water stress, nutrient shortage and day length also change development, so treat stage dates as estimates and scout the field. See the crop calendar for typical Indian sowing windows and the temperature converter for single readings.
Growing Degree Days Calculator FAQ
How do you calculate growing degree days?
Add the day’s high and low temperature, divide by 2 and subtract the crop’s base temperature. If the result is negative, the day counts 0. Sum the days since planting.
How many GDD does corn need?
Most corn belt hybrids need about 2,400–2,800 °F GDD from planting to black layer. A 2,700-GDD hybrid silks at about 1,400 GDD.
What is the 86/50 rule?
For corn, daily maximums above 86 °F are counted as 86 and minimums below 50 °F as 50 before averaging, because growth does not increase above or below those limits.
What is a DD60?
A degree day with a 60 °F base, used for cotton: (Tmax + Tmin) ÷ 2 − 60. About 50 DD60s bring cotton to emergence and about 940 to first bloom.
Can I use Celsius?
Yes. Choose °C; the calculator converts the base, thresholds, your temperatures and the stage targets (°C-days = °F-days ÷ 1.8).
Is this growing degree days calculator free?
Yes, completely free, with no sign-up. It runs in your browser and does not upload your weather data.