Hg porosimetry determines the pore volume based on the volume of mercury injected into a specimen under increasing pressure. The volume of mercury that is pressed in directly gives the pore volume that is reached. With increasing pressures, smaller and smaller pore radii are reached. According to the Washburn equation (Washburn 1921) it is possible from this relationship to calculate - for a non-wetting fluid such as Hg - the reached pore size for each presure level an consequently a frequency distribution of the pore radii:
D = (1/P)*4γ*cosφ
where D is pore diameter, P is the applied pressure, γ the surface tension of mercury, and φ the contact angle between the mercury and the sample.