  We propose a simple approximation scheme to compute the effective charge of
highly charged colloids (spherical or cylindrical with infinite length). Within
non-linear Poisson-Boltzmann theory, we start from an expression of the
effective charge in the infinite dilution limit which is asymptotically valid
for large salt concentrations; this result is then extended to finite colloidal
concentration, approximating the salt partitioning effect which relates the
salt content in the suspension to that of a dializing reservoir. This leads to
an analytical expression of the effective charge as a function of colloid
volume fraction and salt concentration. These results compare favorably with
the effective charges {\em at saturation} (i.e. in the limit of large bare
charge) computed numerically following the standard prescription proposed by
Alexander {\it et al.} within the cell model.
