Growing Degree Days (GDD/GDU) Calculator

Daily Details

How Growing Degree Days Works

Growing degree days estimate accumulated useful heat. The basic idea is simple: average the daily minimum and maximum temperature, subtract the base temperature, and add the positive daily values over time. The base temperature is the temperature below which the crop, insect, or biological process is assumed to develop very slowly or not at all.

Some models use a modified method. Corn GDU is often calculated with the 86/50 method: lows below 50°F are set to 50°F and highs above 86°F are capped at 86°F before averaging. Pest models may use different lower and upper developmental thresholds, so the values from a local extension guide or product label should override generic presets.

Crop And Pest Reference Values

Use these as starting points only. Local extension guidance, pest model instructions, variety recommendations, and crop consultants should override generic base and cutoff values.

Method Choices

Simple average: Daily GDD = ((Tmax + Tmin) / 2) - base. This is useful for checking raw textbook math, but it can produce negative days.

Modified lower + upper bounds: Tmin is raised to the base and Tmax is capped at the upper cutoff before averaging. Use this for corn 86/50 and models that explicitly set both bounds.

Lower-bound-only: Tmin is raised to the base before averaging, but hot days are not capped. Use only when the model specifies a lower threshold but no upper cutoff.

No-negative-GDD: Raw daily average minus base is floored at zero. Use this when a model accumulates positive heat units without changing the daily min/max inputs.

Worked Examples

Methodology And Sources

This calculator uses daily minimum and maximum air temperatures entered by the user. It does not fetch weather station data. Results are planning estimates and should not replace scouting, local extension guidance, variety-specific thresholds, or field diagnosis.

Method notes are based on university and climate references that describe accumulated degree-days, biological lower and upper thresholds, biofix dates, unit conversion, and the fact that calculation methods differ by model.

• University of California IPM: Degree-days concepts and calculation methods
• Midwestern Regional Climate Center: Growing degree day background
• North Dakota Agricultural Weather Network: Corn growing degree days

Updated: June 29, 2026.

FAQs

How do you calculate growing degree days?

The common simple formula is daily GDD = ((daily maximum temperature + daily minimum temperature) / 2) - base temperature. Many crop and pest models then floor negative values at zero and may adjust temperatures below the base or above an upper cutoff before averaging.

What base temperature should I use?

Use the base temperature specified for the crop, insect, disease model, or extension recommendation you are following. Generic starting points are about 50°F or 10°C for many warm-season crops, about 40°F or 5°C for cool-season crops, and crop-specific values for pest models.

What is the 86/50 method?

The 86/50 method is a modified GDD method commonly used for corn. Minimum temperatures below 50°F are raised to 50°F, maximum temperatures above 86°F are capped at 86°F, and the adjusted average is compared with 50°F.

Can GDD be negative?

Most production and pest-tracking models do not subtract heat units from the seasonal total, so daily GDD is floored at zero. A raw simple-average method can produce a negative number, but that is usually not accumulated unless a specific model says to do so.

When should accumulation start?

Start from the biological event used by the model, such as planting date, emergence, first trap catch, first bloom, January 1, or another extension-defined biofix date.

What is the difference between GDD and GDU?

GDD means growing degree days and GDU means growing degree units. In many row-crop contexts they refer to the same accumulated heat-unit concept, though some crop heat unit systems use specialized formulas.

Should I use Fahrenheit or Celsius?

Use the unit system used by the recommendation or threshold you are comparing against. Fahrenheit and Celsius GDD values are not numerically identical; 9 Fahrenheit degree-days equal 5 Celsius degree-days.

Why does my result differ from an extension tool?

Differences usually come from method choice, upper cutoff handling, lower-threshold handling, weather station location, hourly versus daily data, rounding, and the start date used for accumulation.

How are pest degree days different from crop GDD?

The math is similar, but pest models often use insect-specific lower and upper developmental thresholds and a biofix event such as first trap catch. Crop models usually start from planting, emergence, or a seasonal date.

Disclaimer

GDD is a useful biological index, but crop stage also depends on stress, genetics, moisture, stand quality, planting depth, sensor quality, and the model used. Treat results as planning estimates.