Abstract A calculation code, named INLUX, able to predict the average illuminance on the inside surfaces of a room with six walls and a window, is presented. The walls and the window are subdivided into a given number of small areas, identified by the coordinates of their centres. The code solves, at each desired instant, the system of the illuminance equations of each surface element, characterized by its reflection coefficient and its view factors toward the other elements. It is assumed that all surfaces reflect the light in a diffuse manner. The model belongs to the calculation models classified as “radiosity models” The code was validated by comparing the calculated values of illuminance with the experimental values measured inside a scale model (1:5) of a building room, located outdoors, in overcast sky conditions. The validation, performed using the sky luminance data measured by a sky scanner as input data, showed a good performance of the code.