The void ratio (e) of a mixture of solids and fluids (gases and liquids), or of a porous composite material such as concrete, is the ratio of the volume of the voids (V_V) filled by the fluids to the volume of all the solids (V_S). It is a dimensionless quantity in materials science and in soil science, and is closely related to the porosity (often noted as \phi, or {\eta}, depending on the convention), the ratio of the volume of voids (V_V) to the total (or bulk) volume (V_T), as follows: in which, for idealized porous media with a rigid and undeformable skeleton structure (i.e., without variation of total volume (V_T) when the water content of the sample changes (no expansion or swelling with the wetting of the sample); nor contraction or shrinking effect after drying of the sample), the total (or bulk) volume (V_T) of an ideal porous material is the sum of the volume of the solids (V_S) and the volume of voids (V_V): (in a rock, or in a soil, this also assumes that the solid grains and the pore fluid are clearly separated, so swelling clay minerals such as smectite, montmorillonite, or bentonite containing bound water in their interlayer space are not considered here.) and where e is the void ratio, \phi is the porosity, VV is the volume of void-space (gases and liquids), VS is the volume of solids, and VT is the total (or bulk) volume. This figure is relevant in composites, in mining (particular with regard to the properties of tailings), and in soil science. In geotechnical engine****ering, it is considered one of the state variables of soils and represented by the symbol e. Note that in geotechnical engineering, the symbol \phi usually represents the angle of shearing resistance, a shear strength (soil) parameter. Because of this, in soil science and geotechnics, these two equations are usually presented using {\eta} for porosity: and where e is the void ratio, {\eta} is the porosity, VV is the volume of void-space (air and water), VS is the volume of solids, and VT is the total (or bulk) volume.

Applications in soil sciences and geomechanics