The correct quantification of porosity is essential in all studies pertaining to porous media. A host of existing works employs a constant, bulk value for porosity, even when the porous sample is attached to a free fluid. Since the volume fraction of the solid matrix near the interface region differs from that in the core, the porosity undergoes a spatial variation. Here we present a novel relation for the porosity as a function of depth below the interface, using the concept of surface roughness applied on the classical definition of open porosity. This relation has been verified by computational modeling as well as non-invasive laser experiments. It has been shown that this depth-dependent porosity relation applies also to a non-granular porous layer such as a biofilm. (C) 2014 Elsevier Ltd. All rights reserved.