GDU Calculator — Growing Degree Units

Compute a day's Growing Degree Units from the daily high and low temperature and a crop base temperature, using GDU = (Tmax + Tmin)/2 − Tbase.

Quick Facts

Formula
GDU = (Tmax + Tmin) / 2 − Tbase, never below 0
Base temperature is crop-specific; 50°F is standard for corn.

Your Results

Calculated
Growing Degree Units (today)
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Heat accumulated this day
Mean temperature
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(Tmax + Tmin) / 2, after any caps
Base temperature used
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Threshold below which growth stops

Ready

Enter the day's high, low, and base temperature, then calculate.

About the GDU Calculator — Growing Degree Units

Growing Degree Units (GDU), also called Growing Degree Days (GDD) or heat units, measure the accumulated warmth a crop experiences over a day. Plant development is driven far more by temperature than by the calendar: a corn hybrid does not mature "in 90 days" so much as after it has banked a fixed amount of heat. GDU turns each day's temperatures into a single number of growth-driving heat, so growers can predict emergence, silking, black layer, and harvest more reliably than counting days on a calendar.

The formula

For a single day, the standard formula is:

GDU = ( Tmax + Tmin ) / 2 − Tbase

You take the average of the day's high and low temperature, then subtract the crop's base temperature — the threshold below which that crop makes essentially no growth. If the result is negative (a cold day whose average is below the base), GDU is set to 0, because development does not run backward. To track a whole season, add up the daily GDU from planting to the present day; that running total is what you compare against a hybrid's GDU-to-maturity rating.

The corn 86/50 "modified" method

Corn (maize) uses a widely adopted refinement. Before averaging, the daily high is capped at 86°F and the daily low is raised to 50°F. The reasoning is biological: corn growth does not accelerate above roughly 86°F, and it effectively halts below 50°F, so an unusually hot afternoon or a cold night should not inflate or deflate the day's heat units. Turn on "Corn 86/50 caps" above to apply this method; leave it off for the simple average used by many other crops and for general degree-day work.

Base temperatures for common crops

  • Corn (maize): 50°F (10°C) — the reference case, with the 86/50 caps.
  • Soybeans: 50°F (10°C).
  • Wheat, oats, barley (cool-season): about 32–40°F (0–4.4°C).
  • Cotton, sorghum, and many warm-season crops: 60°F (15.6°C).
  • Grapevines: 50°F (10°C), the basis of the Winkler/Amerine climate index.

Base temperature is a property of the crop, not the weather, so choose it from a reliable extension source for the species you are tracking.

Why growers use it

Because GDU accumulation reflects real heat rather than elapsed time, it predicts crop stages across seasons and locations far better than day counts. A cool spring delays a calendar prediction but is captured automatically by slower GDU accumulation. Field corn hybrids are commonly rated around 2,400–2,800 GDU from planting to maturity (base 50°F), so a running total lets a grower estimate weeks ahead when a field will reach a given stage — useful for scheduling nitrogen, scouting for pests whose life cycles are also GDU-driven, and planning harvest and grain drying.

Frequently Asked Questions

Do I enter temperatures in Fahrenheit or Celsius?
Either — but the high, low, and base temperature must all be in the same unit, and the resulting GDU are in that unit. The defaults here (base 50°F, caps of 86°F/50°F) are the Fahrenheit corn convention. In Celsius the equivalent corn values are base 10°C with caps of 30°C and 10°C. Note that a GDU total in °F units is 1.8× the same total in °C units, so never mix ratings and readings across scales.
What happens on a cold day when the average is below the base?
The daily GDU is floored at zero. A negative value would imply the crop lost development, which does not happen — the plant simply makes no progress that day. This calculator reports 0 GDU in that case rather than a negative number.
Should I use the 86/50 caps?
Use them for corn, where they are the standard. For most other crops the simple average (caps off) is used, though some warm-season crops apply only an upper cap. When in doubt, match the method used by the hybrid or extension rating you are comparing against, since the cap choice changes the accumulated total.

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