Here, it can be noted that the apparent permeability is easily calculable from the intrinsic permeability by taking into consideration the fraction of non-ionized molecules (from pKa) and the permeation rate through an unstirred water layer, which is specific to the experimental setup and can be determined from data for fast-permeating molecules. Conversely, the determination of intrinsic permeability from the apparent one is only feasible when the apparent permeability is not close to the diffusion limit. This relation limits the usefulness of published apparent permeability data for intrinsic permeability models.