Porosity is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100% . Porosity is used in geology, hydrogeology, soil science, and building science to describe the fraction of void space in a porous medium, such as rock or sediment, where the void may contain air or water. Porosity is defined by the ratio of the volume of void-space (such as fluids) to the total or bulk volume of material, including the solid and void components. Porosity is a fraction between 0 and 1, typically ranging from less than 0.005 for solid granite to more than 0.5 for peat and clay. Porosity is controlled by rock type, pore distribution, cementation, diagenetic history, and composition. However, porosity is not controlled by grain size, as the volume of between-grain space is related only to the method of grain packing.
Porosity is an important parameter in groundwater studies where it is used to estimate storage and travel times in aquifers and aquitards. Density is commonly used in gravity surveys to help determine the thicknesses and variation of different rocks. Porosity is also used to describe the volume of pores or interstices of a material in geology and engineering. In addition, porosity is important in the study of hair, where it refers to the way in which hair absorbs water.
