GDU Calculator
Growing Degree Units (GDD) for corn, soybean, wheat, alfalfa, sorghum & cotton — accumulate daily heat, track growth stages, forecast maturity.
1. Crop & temperature settings
Preset source: UNL CropWatch; Purdue Extension (Bob Nielsen)
2. Single-day GDU
3. Multi-day accumulation (paste CSV)
| Date | High | Low | Daily GDU | Cumulative |
|---|
4. Growth-stage tracker (corn)
Targets are estimates — they vary by hybrid, region and conditions.
Methodology
Formula (simple average method): cap the daily high at the ceiling, floor the daily low at the base, then GDU = ((high + low) / 2) − base. Results ≤ 0 count as 0 and are never subtracted. Example: high 85°F, low 65°F, base 50°F → 25 GDU.
This calculator uses the average method, the standard for corn per UNL CropWatch. It does not use the single-sine or triangle methods — those model the day's temperature curve and can give slightly different daily values.
Base/ceiling sources: corn — UNL CropWatch, Purdue Extension (Bob Nielsen), Bayer; soybean/sorghum/cotton/wheat — Oklahoma Mesonet; alfalfa — UMN Extension, Oklahoma Mesonet. Stage targets: emergence ~90–125 (UNL/Iowa/UW), maturity 2,400–2,800 for 105–115-day hybrids (Farm Progress/Purdue) — all approximate.
Frequently asked questions
What is GDU (Growing Degree Units)?
Growing Degree Units (GDU), also called Growing Degree Days (GDD), measure the heat a crop accumulates over time. Because plant development is driven by temperature rather than the calendar, GDUs predict growth stages — emergence, flowering, maturity — more reliably than counting days. GDU and GDD refer to the same calculation (UNL CropWatch).
How is daily GDU calculated?
This calculator uses the standard average method: cap the daily high at the crop's ceiling (86°F / 30°C for corn), floor the daily low at the base temperature (50°F / 10°C for corn), then compute ((high + low) / 2) − base. Example: a day with high 85°F and low 65°F gives ((85 + 65) / 2) − 50 = 25 GDU. Days that come out at or below zero count as 0 and are never subtracted from the total.
Why is the high temperature capped at 86°F (30°C) for corn?
Corn's growth rate does not increase beyond about 86°F (30°C) — extra heat mainly causes heat stress, not faster development (UNL CropWatch; Purdue Extension). Capping prevents a 100°F scorcher from counting as more growth than an 86°F day. Example: high 95°F, low 72°F → ((86 + 72) / 2) − 50 = 29 GDU, not 33.5. Other crops have different ceilings: soybean 95°F (35°C), cotton 100°F (37.8°C).
What base temperature should I use for my crop?
The base temperature is the point below which the crop essentially stops developing. Verified presets: corn 50°F (10°C), soybean 50°F (10°C), wheat 32°F (0°C), alfalfa 41°F (5°C), sorghum 55°F (12.8°C), cotton 60°F (15.6°C). Sources: UNL CropWatch and Purdue Extension for corn; Oklahoma Mesonet for the rest; UMN Extension for alfalfa. You can also enter any custom base and ceiling. Note: some Canadian sources use 5°C for cereals instead of 0°C — if your local extension recommends a different base, enter it as custom.
How many GDUs does corn need from planting to maturity?
Most 105- to 115-day relative-maturity hybrids need roughly 2,400 to 2,800 GDUs from emergence to black layer (physiological maturity), per Farm Progress citing Purdue Extension corn specialist Bob Nielsen. Typical stage targets (base 50°F, all estimates — they vary by hybrid and region): VE emergence ~120, V6 ~475, VT tasseling ~1,350, R1 silking ~1,400, R6 black layer ~2,700. Late-planted corn often matures with somewhat fewer GDUs.
When do I start accumulating GDUs?
Start the day after planting and add each day's GDU to a running total through harvest (Bayer sweet-corn guide). Emergence typically occurs around 100–120 accumulated GDUs after planting (UNL CropWatch: 90–120; Iowa research: 100–120; University of Wisconsin: ~125), assuming adequate soil moisture.
Can daily GDU be negative?
No. When the day's average is at or below the base temperature, that day contributes 0 — cold days are ignored, never subtracted from the cumulative total (Cornell Cooperative Extension). Example: high 45°F, low 30°F with a 50°F base gives ((45 + 50) / 2) − 50 = −2.5, which the calculator records as 0 GDU.
What method does this calculator use — average, single-sine, or triangle?
The simple average method with cap and floor adjustments, which is the standard for corn (UNL CropWatch). Single-sine and triangle methods model the day's temperature curve instead of averaging high and low; they usually give similar totals over a season but can differ on individual days. Because weather stations and universities may use different methods, compare your totals only against sources using the same method — method mismatch is the most common reason GDU totals disagree.
How accurate is the stage forecast?
The stage tracker is an estimate, not a prediction of what you will see in the field. It compares your cumulative GDU against typical stage targets and projects remaining days using your recent average daily GDU. Example: with 1,450 GDU accumulated you are at approximately R1 (silking); with 100 GDU still needed and a recent average of 25 GDU/day, about 4 more days. Real development also depends on moisture, nutrients, pests, and stress.
What are the limits of GDU as a tool?
GDU measures heat only. It is an excellent predictor when heat is the only limiting factor — with ample water and nutrients and no pest pressure (UMN Extension) — but drought, flooding, nutrient deficiency, disease, and extreme heat stress can all push real development off the GDU schedule. Stage targets also shift between hybrids, planting dates, and regions, so treat every threshold as approximate and confirm with scouting.
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