Runoff Lab is a calculation and analysis tool aimed at professionals who have to deal with hydrological computations. Runoff Lab makes elaborations such as probabilistic interpretation of pluviometric series, calculation of pluviometric probability curves, definition of synthetic pluviograms, and inflow-deflow transformation extremely simple and intuitive. The attention given to the design of the user interface allows for an extremely intuitive and easy-to-use program: the use of tree navigation menus, the assignment of relationships between entities by simple drag-and-drop operations, the Copy/Paste of created entities, and the unlimited Undo/Redo series are just some of the features that allow for a high learning curve in the use of the software.