Porosity vs. Permeability vs. Capillarity Porosity represents the STORAGE CAPACITY of the geologic material and is the ratio of the volume of holes/openings to the total volume of the material. (Some people call openings as VOIDS in textbooks.) Porosity = (volume of holes in material) / (total volume of material). The porosity of the same material is the same, even if the particle size is different.

But permeability is a different thing. It increases as particle size increases. By definition, permeability is a MEASURE OF EASE with which fluids will flow though a porous rock, soil or sediment. A material that has high porosity does not have to have high permeability. For example, clay has high porosity but low permeability.

Capillarity, in physics, is a phenomenon caused by surface tension and that causes a series of distortion of liquid surface. But in earth science, we focus on ELEVATION. When the distance between particles is getting smaller, the liquid (water) has no place to go and have to go up. That means capillarity increase as particle sizes decreases.