Your contractor will use industry standard methods for determining the opening width and height for replacement windows. Since each replacement window opening may be different, no one method is THE correct solution. Generally speaking, three measurements are taken from in the vertical plane, from head to sill, and in the horizontal plane, from jamb to jamb. The shortest dimension in each plane is used to determine the rough opening (RO) for the replacement window. When RO measurements are provided, Replacement windows will be manufactured ¼ inch smaller in width and height.