The ratio of void space to total volume of a soil or rock material, expressed as a percentage. It indicates the maximum amount of water a material can hold.
Porosity is the fraction of the total volume of a soil, sediment, or rock that consists of void spaces (pores), expressed as a decimal or percentage. Total porosity includes all voids regardless of whether they are interconnected or isolated, while effective porosity considers only the interconnected pore spaces through which fluid can actually flow. Porosity values range from less than 1% in dense crystalline rocks to over 60% in some clays, with typical values of 25-50% for unconsolidated sediments and 10-30% for consolidated sedimentary rocks. Porosity is determined by grain size, sorting, packing, cementation, and the presence of fractures or dissolution features. It is measured in the laboratory by saturating and weighing core samples, or estimated in the field using geophysical methods such as neutron logging. While porosity indicates the total storage capacity, it does not directly indicate how easily water can flow through the material; that property is described by hydraulic conductivity. The distinction between total and effective porosity is particularly important in fine-grained materials like clays, which may have high total porosity but very low effective porosity due to the dominance of micropores that retain water by capillary forces.