How this calculation works
Growing degree days accumulate heat above an explicit base instead of counting every calendar day equally. A warm day and a cold day can contribute different amounts to the same seasonal total. The calculation convention must match the threshold being compared.
Daily GDD = max(0, (used minimum + used maximum)/2 − base); sum entered rows; modified mode clamps each endpoint before averagingA worked example
With a 10 °C base and 30 °C upper cap, the first example day uses ten and twenty for five degree days. The second clamps its five-degree minimum to ten and contributes 7.5. The third day caps its 35-degree maximum at thirty, producing fifteen. The modified total is 27.5 °C days. In uncapped mean mode the same three rows total 27.5 too, but the daily contributions differ; other logs need not give equal totals.
Good to know
Entered observations and explicit daily conventions only; no weather retrieval, missing-day inference, crop recommendation or phenology forecast. Use a threshold developed for the same base, cap, units and method. A numerically precise sum can still be incompatible with a threshold using another convention.
Reference: Montana Climate Office — degree day conventions (opens in a new tab) ↗
What the model answers
Growing degree days accumulate heat above an explicit base instead of counting every calendar day equally. A warm day and a cold day can contribute different amounts to the same seasonal total. The calculation convention must match the threshold being compared.
This page offers a simple daily mean and a modified endpoint-clamping method. They are not interchangeable with single-sine integration or every crop-specific implementation. The chosen method is shown in the used-temperature table.
The Crop water ET0 depletion schedule calculator uses evapotranspiration and soil-water assumptions. Heat-unit accumulation is a development index and cannot substitute for an ET0 water-demand input.
Measure and enter consistently
Enter daily minimum and maximum in Celsius, with minimum no greater than maximum. Labels identify observations; the tool does not infer dates, verify consecutive days or fill a missing weather record.
Modified mode clamps each endpoint between base and upper cap before averaging. Uncapped mean mode uses the original endpoints and only clips the final daily heat unit at zero. The upper-cap input has no role in that uncapped convention.
Interpret the result
Cumulative heat units are degree differences multiplied by days, not an ordinary temperature. Celsius and Fahrenheit degree days differ by a factor of 1.8 when otherwise defined consistently; adding a 32-degree temperature offset is not the conversion.
Remaining units compare the entered total with a supplied threshold. They do not forecast when that threshold will be reached, recommend a crop threshold or prove a plant has reached a developmental stage.
Work through the numbers
With a 10 °C base and 30 °C upper cap, the first example day uses ten and twenty for five degree days. The second clamps its five-degree minimum to ten and contributes 7.5.
The third day caps its 35-degree maximum at thirty, producing fifteen. The modified total is 27.5 °C days. In uncapped mean mode the same three rows total 27.5 too, but the daily contributions differ; other logs need not give equal totals.
Where this model stops
Entered observations and explicit daily conventions only; no weather retrieval, missing-day inference, crop recommendation or phenology forecast.
Use a threshold developed for the same base, cap, units and method. A numerically precise sum can still be incompatible with a threshold using another convention.
A few questions, answered
Are day labels treated as a calendar?
No. Every row is one entered observation and gaps are not filled.
Why do methods sometimes disagree?
Clamping endpoints before averaging differs from clipping only the final average above base.