Void Volume (VV), typically expressed as a percent, can be defined as the ratio of the porous material's air volume (VA) to the porous material’s total volume (VT). For example, fifty ping pong balls inside a two-gallon container may take up 60% of the total amount of space in that container. The space remaining may make up 40% of that container. In this case, pore (or void) volume is equal to the ratio of the air volume of the container to the total volume of the container. Similarly, the pore volume of a porous material is the ratio of that material's air volume to total volume (VV % = VA/VT).
Typically, porosimeters are used to measure both the pore volume and pore size distribution in porous materials. They can use either liquid intrusion into pores or liquid extrusion from pores to measure pore volume. Porosimeters or bubble point testers can be used to determine the point at which pressure can overcome capillary action in a porous material.