While the calculation of the total resistance of two resistors in parallel is simple, this code shows you how to trap a number of potential errors that come with the entry of the required data. All is wrapped easily into a colorful GUI program, due to the simplicity of the Tkinter module. The GUI contains two data entry boxes, two labels for user instructions, one label to show the calculated results, and a button to start the calculation defined in the function. This could easily be a GUI template for other calculations that need two data inputs. Remember that Tkinter works with Windows, Linux and the Mac OS-X.