A compression strength theory for concrete based on the capillary porosity of binder paste in concrete and the intrinsic tensional strength of the CSH gel is presented. The capillary pores were modelled as spheres of equal radius and the tensional stress peak on the surface of the voids was calculated using a FEM program. The intrinsic tensional strength of the binder was determined from gas evolution curves of the binder paste by thermogravimetric means. To validitate the theory, eleven different test concretes with compression strength values from 25 to 135 MPa were produced. As binders, five different Portland cement types with and without silica fume were used. The testing ages of the concretes were 28 and 365 days. The porosities of the test concretes were measured by mercury porosimetry, and the microporosity of the hardened binder paste by nitrogen adsorption. The correlation coefficient R-squared between the test results and the results of the theory was 0·941.