The asymptotic behaviour of the ion density distributions at a charged planar interface in contact with a symmetric electrolyte is analysed using the dressed ion theory of Kjellander and Mitchell. This gives a rigorous way of defining an effective surface charge for such a double layer, ie the surface charge that an ion "sees" at a large distance from the interface. Numerical calculations of this effective surface charge are performed using the anisotropic hypernetted chain approximation for 1:1 and 2:2 electrolytes over a wide range of surface charges. For monovalent ions we observe a saturation of the effective charge when the real charge is increased-except for high concentrations and large ions. For divalent ions we observe a reversal in the sign of the effective surface charge as the actual surface charge increases. This reversal is related to an over-compensation of the bare surface charge in the first layer of counterions and the formation of an adjacent colon layer.