) 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 (
) filled by the fluids to the volume of all the solids (
It is a dimensionless quantity in materials science and in soil science, and is closely related to the porosity (often noted as
, depending on the convention), the ratio of the volume of voids (
), as follows: in which, for idealized porous media with a rigid and undeformable skeleton structure (i.e., without variation of total volume (
) of an ideal porous material is the sum of the volume of the solids (
): (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.)
This figure is relevant in composites, in mining (particular with regard to the properties of tailings), and in soil science.
In geotechnical engineering, it is considered one of the state variables of soils and represented by the symbol
Because of this, in soil science and geotechnics, these two equations are usually presented using