The bubble point method is the most widely used for pore size determination. It is based on the fact that, for a given fluid and pore size with a constant wetting, the pressure required to force an air bubble through the pore is inverse proportion to the size of the hole.

The theory of capillarity states that the height of a water column in a capillary is indirectly proportional to the capillary diameter.

Surface tension forces held up the water in the capillary and as its diameter gets smaller, the weight in the water column get higher. Water can be pushed back down in a pressure wich has the same equivalent height as that of the water column. Thus by determining pressure necessary to force water out of the capillary, the diameter of the capillar can be calculated.

In practice, the pore size of the filter element can be established by wetting the element with the fluid and measuring the pressure at which the first stream of bubbles is emitted from the upper surface of the element.