To design a passive building a careful assessment of the direct solar gain is required, so that the amount of solar energy collected by transparent surfaces can be determined. With this aim a computer package has been developed by means of which an hourly computation of the solar beam radiation coming into the building through oriented windows can be performed. The program also obtains the solar energy absorbed on the interior walls, floor and ceiling. The procedure followed is based on solar geometric relations and takes into account the transmission coefficient of the glass and the absorbtance of the walls.