Calculating Growing Degree Days

The growth and development for corn is dependent on temperature and Growing Degree Days (GDD) are often used to determine the accumulated heat units.  The most commonly used method is the Modified Growing Degree Day formula.  The modification is the limit on minimum and maximum (below 50 F and above 86 F) temperature, with the assumption that corn development is limited when temperatures are below 50 F and above 86 F.  When the daily maximum temperature is above 86 F it is reset to 86 F, likewise if the daily minimum temperature is below 50 F, it is reset to 50 F.
Here is the formula:
((Tmax + Tmin)/2)-50=GDD
Now, you have to think back to elementary school and remember the order of operations.  Start with the operations within the parenthesis, then move outward.
Example: Tmax = 82 F   Tmin=63
((82+63)/2)-50=23 GDD
Example 2:   Tmax=93 F   Tmin = 54 F  (Reverts to max of 86 F)
((86+54)/2)-50=20 GDD
If you have a maximum and minimum temperature recording device, you can calculate the GDD for your location, or you can go to weather.gov, click on your location,  and click on "3 day history" to see daily max and min temperatures.
GDD can also be found at cropwatch.unl.edu under the "weather" tab.
Useful 2 Usable also has a great CornGDD tool that monitors GDD accumulation and corn growth stage.  The tool can be found here.