In petroleum engineering, the Leverett J-function is a dimensionless function of water saturation describing the capillary pressure, where S_w is the water saturation measured as a fraction, p_c is the capillary pressure (in pascal), k is the permeability (measured in m²), \phi is the porosity (0-1), \gamma is the surface tension (in N/m) and \theta is the contact angle. The function is important in that it is constant for a given saturation within a reservoir, thus relating reservoir properties for neighboring beds. The Leverett J-function is an attempt at extrapolating capillary pressure data for a given rock to rocks that are similar but with differing permeability, porosity and wetting properties. It assumes that the porous rock can be modelled as a bundle of non-connecting capillary tubes, where the factor is a characteristic length of the capillaries' radii. This function is also widely used in modeling two-phase flow of proton-exchange membrane fuel cells. A large degree of hydration is needed for good proton conductivity while large liquid water saturation in pores of catalyst layer or diffusion media will impede gas transport in the cathode. J-function in analyzing capillary pressure data is analogous with TEM-function in analyzing relative permeability data.