Growing Degree Days Calculator (GDD)

This calculator works out growing degree days (GDD) for one day, the heat measure used to time planting, insect activity and crop stages. Enter the day’s high and low temperatures and the base temperature; you get the GDD in Fahrenheit units, the mean temperature and the GDD in Celsius units.

Formula

GDD = (high + low) ÷ 2 − base, and 0 if that is negative (the simple average method). GDD in Celsius units = GDD in Fahrenheit units × 5 ÷ 9. Add the daily values over the season for the running total.

• tx, tn: the day’s high and low air temperatures in °F (the high cannot be below the low)
• b: the base temperature below which the plant or insect does not develop, from the model you are following (50 °F is common for many crops)

Worked Example

A day with a high of 82 °F and a low of 58 °F, base 50 °F.

1. Mean = (82 + 58) ÷ 2 = 70.0 °F
2. GDD = 70 − 50 = 20.0
3. In Celsius units: 20 × 5 ÷ 9 = 11.1

How to use the result

Keep a running total from the start date your model names; crop and pest guides list the GDD at which stages occur. Use the same base and method as the guide, or the totals will not match.

Limits and common mistakes

• Some models cap the high (for example at 86 °F for corn) and raise the low to the base before averaging; this calculator uses the plain average.
• Do not mix Fahrenheit and Celsius totals: they differ by the 5/9 factor.
• Use the air temperature recorded at a nearby station or your own shaded thermometer.

Frequently Asked Questions

What base temperature should I use?

The one your crop or pest model states; 50 °F is common, but some use 32 °F or 41 °F.

Can growing degree days be negative?

No, a day with a mean below the base adds 0.

How do I convert GDD from °F to °C?

Multiply by 5/9; this equals the °C GDD with the matching base (50 °F = 10 °C).

What is the mean temperature method?

It averages the high and the low and subtracts the base, as here.

in tx | High temperature | °F | 82 | -459.67 | 150
in tn | Low temperature | °F | 58 | -459.67 | 150
in b | Base temperature | °F | 50 | -100 | 120
out g | Growing degree days | GDD (°F) | max(0, (tx + tn) / 2 - b) + 0 * sqrt(tx - tn) | 1
out tmean | Mean temperature | °F | (tx + tn) / 2 | 1
out gc | Growing degree days | GDD (°C) | g * 5 / 9 | 1