The evaluation of the creep concrete effects for the check of slender r.c. columns can be obtained with the creep eccentricity method, which expresses the increase of the displacement due to the creep by means of a simple formula. This formula is deduced for homogeneous material with linear viscoelastic behaviour and leads generally to very conservative results, because does not take into account the reinforcements, which reduce the viscous deformations. In this work it is derived a modified formula for the additional eccentricity evaluation, different from others referred in bibliography, which takes into account the reinforcements and gives a value of the additional eccentricity depending also on the homogenized section inertia. From some comparisons with more exact methodologies it is deduced that the proposed formulation is satisfying, because it reduces strongly the conservativity of the original method, preserving the characteristic of simple application.