This study derives and validates an empirical effort estimation model for graphical user interface (GUI) systems. It investigates the relationship between the effort to code and unit test GUI systems and the numbers of different types of widgets (e.g., text boxes, check boxes, list boxes) designed in those systems. The study focuses on systems implemented using Visual Basic, a popular Microsoft Windows programming language. The GUI effort estimation model was empirically derived from an agent diary system implemented in an international bank. The model exhibited a strong relationship (R2 = 0.863, p < 0.001) between estimated effort and the numbers of different types of widgets. In an exploratory test of robustness, the GUI effort estimation model was applied without calibration to a different development environment. As expected, there was a high error level, confirming the need for calibration when applying effort estimation models to foreign settings.