Donaldson R F Growing Degree Days (GDD) Calculator
Work out cumulative Growing Degree Days from a list of daily high and low temperatures, using the standard average method.
How it is worked out
For each day, GDD = max(0, (high + low) ÷ 2 − base temperature) — the "average method", the most common simple way of calculating growing degree days. Days where the average temperature is below the base temperature contribute zero (plants don't grow in reverse). The base temperature depends on the crop — 10°C (50°F) is widely used for crops like maize and many vegetables, but check the figure for what you're growing.
Source: the standard "average method" for Growing Degree Days, as documented by agricultural extension services and widely used in crop-development tracking.