Deformation of a cylinder of yield stress material under gravitational loading is investigated numerically, and found to obey a geometric scaling law that is independent of both the magnitude of deformation and yield stress. This scaling law provides a fundamental connection between the initial and final dimensions of the body, and the critical aspect ratio (radius/height) for the onset of flow. Importantly, this finding is contrary to the standard model of [Murata, Mater. Constr. (Paris) 17, 117-129 (1984); Pashias et al., J. Rheol. 40, 1179-1189 (1996)], which is currently used in industrial processes to measure the yield stress, and predicts that the deformation is independent of radius. Consequently, the accuracy of the latter model is assessed, and a new formalism for such yield stress measurements proposed. (C) 2005 The Society of Rheology.