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HORT410 - Vegetable Crops

Heat Unit Calculator

The growth and development of both plants and insects is strongly temperature dependent. Their temperature-dependent growth and development is often described as requiring a certain number of "heat units". Daily heat units are calculated as the difference between the average of the daily maximum and minimum temperatures and a certain critical basal or developmental "threshold" temperature. The threshold temperature varies between species. For corn the threshold temperature is 50oF. For peas the threshold temperature is 40oF. The daily heat unit values can be summed for the season to derive cumulative seasonal heat units. This is more predictive of plant and insect growth and development than time alone.

The following program calculates and displays Heat Units for the numerical values of daily Maximum, Minimum and Threshold temperatures that you enter below. The routine uses the following simple equation:

Heat Units = ((Maximum Temp. + Minimum Temp.)/2) - Threshold Temp.

After entering the Maximum, Minimum and Threshold temperatures, click the Calculate button.

Enter Daily Maximum Temperature (oF):

Enter Daily Minimum Temperature (oF):

Enter Threshold (Basal) Temperature (oF):


Computed Heat Units =

Cumulative Heat Units =

To convert a temperature from oC to oF, enter a numerical value of the temperature in the oC textbox and then press the Convert button. The temperature in oF will be displayed using the formula: Temperature oF = 9/5(oC) + 32.

Convert Temperature (oC): to Temperature (oF):

David Rhodes
Department of Horticulture & Landscape Architecture
Horticulture Building
625 Agriculture Mall Drive
Purdue University
West Lafayette, IN 47907-2010
Last Update: 8/20/03