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
Example 2: Tmax=93 F Tmin = 54 F (Reverts to max of 86 F)
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.