Growing Degree Day Calculator

Accumulate temperature units using a specified growing-degree-day model.

Formula, inputs and conditions

Formula: max(0,(min(max(Daily high temperature,Model base temperature),Model upper temperature)+min(max(Daily low temperature,Model base temperature),Model upper temperature))/2-Model base temperature)

Decimal results are displayed to two places when needed.

Inputs: Daily high and low temperatures and the base and upper temperatures for the intended model.
Outputs: Daily degree days in °F·day.
Conditions and scope: This tool calculates one day at a time using a capped two-temperature model. Sum daily results for a documented date range. Base, cap, and crop threshold must match the selected model; 50°F is not treated as universal. Results are planning estimates, not engineering, permit, code or contractor-price decisions.

Daily heat accumulation under a chosen model

Growing degree days summarize a day’s temperatures relative to a model base. This tool bounds the high and low within the entered model range, averages them and subtracts the base, with no negative accumulation. Different crops and methods can use different rules.

Measure and calculate

Enter the measured daily high and low in Fahrenheit.
Set the base and upper limit for the chosen model; the upper limit must exceed the base.
Read the result as one day’s modeled heat accumulation, not a maturity forecast.

Calculation method

Daily GDD = maximum of zero and (average of the bounded daily high and low − base).

A worked example

Inputs: The example day has an 80°F high and 60°F low, with a 50°F base and 86°F upper limit.
Calculation: Both temperatures are within range. (80 + 60) ÷ 2 − 50 = 20°F·day.
Result: 20 °F·day

The result is 20 degree days for this date and model. A season total requires additional daily observations; this calculator does not fetch weather or predict a harvest date.

Read the result

Output interpretation and next steps
Output or check	What it means for your project
One day: °F·day	The result is daily heat accumulation under the entered base, upper limit and bounding method. It is not a season total.
Model consistency	Sum daily results only when they use the same model. A harvest or pest-development threshold also needs a matching model and starting date.

Common questions

Can I compare totals from different temperature models?

Only after accounting for their base, limits and calculation methods. Equal numerical totals can represent different models.

Does a negative-temperature day subtract from the season total?

This implementation floors each daily accumulation at zero. It does not return negative heat accumulation.