Accumulate growing degree days from daily high/low temperatures with a base temperature and optional upper cap.
About this tool
Growing Degree Days Calculator is a free, in-browser tool that turns a run of daily high and low temperatures into accumulated growing degree days (GDD), the heat-unit measure used to predict crop and pest development. Paste one "high,low" pair per line, set a base temperature, and it sums the GDD across all the days. Nothing is uploaded — every figure is computed on your device.
Each day's GDD is max(0, (high + low) ÷ 2 − base). With the simple method the high is capped at the upper threshold you enter (if any) before averaging. The modified method also floors both the high and the low at the base temperature before averaging, which prevents very cold nights from pulling the average below the base. Days that average below the base contribute zero, never negative. The total is the sum over every day, and the tool also shows the day count and average GDD per day. Base-temperature presets cover corn (50°F), wheat (40°F) and grapes (10°C).
Use it to track a crop toward maturity or to time pest scouting from a biofix date. Enter temperatures in whichever unit you prefer — just make sure the base and cap use the same unit (°F or °C) as your daily readings.
Frequently asked questions
How is a single day's GDD calculated?
It is the day's average temperature minus the base, floored at zero: max(0, (high + low) ÷ 2 − base). A day averaging at or below the base contributes zero rather than a negative value, so GDD only ever accumulates upward.
What is the difference between the simple and modified methods?
Both average the daily high and low, but the modified method first raises any high or low below the base up to the base (and still caps the high at the upper threshold). This stops cold nights from underweighting a warm day and is common for crops like corn.
What base temperature and cap should I use?
It depends on the crop: 50°F is standard for corn, 40°F for wheat and cool-season grasses, and 10°C for grapes. An upper cap (often 86°F for corn) reflects that growth stops speeding up in extreme heat. Keep base, cap and your temperatures in the same unit.
Is my temperature data uploaded?
No. The daily temperatures you paste and the resulting GDD totals are computed entirely in your browser and never sent to a server, so the tool works offline.