We report the static (ε0) and high frequency (ε∞) dielectric constants of haematite (α-Fe2O3) as calculated using density functional perturbation theory. The values for bulk iron oxide are ε0 11 = 26.41; ε0 33 = 17.84; ε∞ 11 = 7.87 and ε∞ 33 = 7.57. The effective static dielectric screening of porous thin-films is predicted in aqueous solutions, using effective medium theory, to range between 33 and 44 depending on the crystal packing density. The large spread in literature values (12-120) can only be explained through surface and micro-structuring effects. These results will be important for interpreting and modelling the materials behaviour, particularly in electrochemical and photoelectrochemical cells.