This paper addresses the problem of finding the radial stress within a powder body undergoing plastic deformation by confined compression, motivated by the compaction test for powdered materials. Small deformation, elastoplastic behaviour is assumed, with porosity as the parameter governing the evolution of material properties. An elliptic yield surface is selected, whose aspect ratio approaches zero as full density is neared, so as to recover the Von-Mises criterion of plastic yield in ductile metals. The results were used to track the radial stresses on a notional compaction process, and applied to the design of an instrumented closed die. In addition, the aspect ratio of the yield surface showed an evolution according to the widely used Heckel porosity-pressure function [6], thus extending the interpretation of this model to a more general stress state.