The effective hydrodynamic radius (L) of a porous medium is defined by the relation L = √8Fkdc, where F is the formation factor and kdc is the dc permeability. Two approximations of this fundamental length are discussed: the diffusional estimate L≈√8DT1 (Avellaneda and Torquato) and the electrical estimate L≈Λ (Johnson et al.). A new simple proof of the universally valid upper bound L≤√8DT1 is given. A comparison of Λ, L and the Kozeny Carman volume-to-surface ratio is made for a class of `corrugated' capillaries, in which several relevant geometric parameters are varied. Finally, we examine the merits of both estimators on microgeometries with a wide distribution of pore sizes.