To assess the daylighting performances of a building, one of the most commonly used quantities is the Daylight factor, which is defined for a given surface element inside the analysed room as the ratio of the inside and outside illuminances under a CIE overcast sky. The Daylight factor consists of three components: the sky component, due to light flux reaching the surface element directly from the sky, the externally and the internally reflected components, respectively due to light flux reflected on external and internal surfaces. To estimate the direct sky component (also called sky factor), analytical methods can be used, based on the luminance distribution of the sky and the window’s geometric properties (dimensions and position in regard to the considered surface element). However, such methods have always been restricted to vertical (lateral) and horizontal (zenithal) windows, requiring heavy approximations to be applied whenever a tilted rectangular opening was considered. In this paper, a generalized method for assessing the sky component is proposed, extending it to rectangular windows of any tilt angle. As a purely analytical approach was found to be inapplicable, it is based on an optimised combination of vertical and horizontal windows situations. To validate the developed methodology, scale model measurements were performed with a sky simulator for two rectangular openings of varying tilt angle (every 15° from vertical to horizontal): the experimental results proved to be in very good agreement with the calculation-based approach.