pressure difference

I’m learning to work with lattice Boltzmann and I am interested in calculating the gas permeability in porous media using the method. The problem arises when I want to calculate the inlet and outlet pressures. In most of the articles they say pressure only have a direct relationship with density and that if I set the density calculation pressure. However, I do not clear how is it calculated? What equations should be used to calculate the pressures? Where in the program are placed?

Please if anyone can help me about it. Thank