The method is applicable to finished down and/or feather, fit for or constituting filled manufactured articles (e.g. anoraks). The filling power is the height of the volume occupied by filling material subjected to a specific pressure, expressed in millimeters. A test specimen, having a specified mass, is placed in a cylinder and the material is aerated by blowing for a set period. Pressure by way of a platen is then applied to the filling material at a constant speed and after a stipulated time the level of the platen is noted.