Abstract Theories of isotropic consolidation for geomaterials with compressible phases require two loading tests: a full-drainage test at constant pore-pressure and a mixture test in which pore-pressure varies. In a no-drainage mixture test, the pore-pressure change falls short of the confining pressure change. In the unjacketed mixture test, the pore-pressure and confining pressure rates are equal. This paper develops a parametric theory that supports extrapolation of unjacketed bulk compressibility from the compressibilities measured in the full-drainage and mixture tests, elaborates and clarifies the isotropic version of the Biot and Willis theory of consolidation and expresses the unjacketed pore compressibilities of several sandstones in a combination of macroscopic and mesoscopic terms. The mesoscopic model includes solid and fluid phase compressibilities and phase-pressure distribution ratios. The coefficient of fluid content is identified as the product of porosity and the pore flux coefficient. The compressibility-induced component of the strain-rate in an unjacketed test is postulated to be an objective measure. This solution provides a framework for geotechnical modelling software that simulate the effect of porosity variation due to consolidation. With this theory, engineers can infer the phase-pressure distribution ratios from the measured Brown-Korringa compressibilities and represent the effects of both meso-heterogeneity and solid-grain rearrangement in constitutive models.