Approximate analytical formulae valid for any porous media with elongated pores are derived for the electro-osmotic coefficient alpha and for the average ionic concentration (n) over bar. A macroscopic Debye-Huckel length (k) over bar (-1) based on (n) over bar is introduced. Simultaneously, the electro-osmotic coefficient alpha is systematically calculated for various media, zeta potentials and electrolyte concentrations by solving the local equations. Numerical results show that (k) over bar (-1) and alpha follow universal curves valid whatever the porous medium; these curves can be approximated by the analytical formulae previously derived. These formulae can be used to provide a priori estimates of the electro-osmotic coefficient. (C) 2007 Elsevier Inc. All rights reserved.