The forecast of maximum stresses on compaction tools is frequently based on the so-called compressibility curves, obtained according to specific standards. The analysis of compressibility curves enables to draw a simple analytical law, to utilize for further developments. The relationship between radial and axial pressure is described. The radial pressure is the design datum for the correct dimensioning of dies. Literature data on the relationship between applied pressure and friction coefficient enables to derive a model linking compact geometry and axial pressures effectively needed to reach specific densities. For part shapes characterized by a discrete extension on height – such as bushings, for instance – the effects of geometry are linked to 2 dimensionless parameters, one of physical nature (product of the pressure ratio multiplied by friction coefficient) and one of geometrical nature (ratio between «vertical» friction surfaces and double of compaction area). These dimensionless parameters enable to draw the «real» compressibility curves, linked to specific geometries. For part shapes characterized by small height – such as thin disks or plates – the effects of geometry again depend on two dimensionless parameters: one of physical nature (ratio between two times the friction coefficient and pressure ratio) and one of geometrical nature (ratio radius/height of the thin disk). Thinner the disk, higher the pressure needed to attain a given density. The theoretical results are compared with experimental data. The agreement between experimental data and forecasts based onthe theoretical approach is good. The study proves that the standard compressibility curves, if uncritically utilized for predicting stresses acting on tools, are unsuitable to predict the stresses really acting at compaction end.