Cell constant correction values (K) for concrete resistivity measurements on cylindrical test samples performed with the four-point Wenner array probe technique are presented. Specimens dimensions correspond to standard cylinder sizes used for compressive strength or rapid chloride permeability tests, as well as typical concrete core sizes. The values of K were determined experimentally and numerically, by means of Finite Element (FE) simulation. The effect of different maximum aggregate sizes and aggregate types on the variability of the resistivity readings was evaluated. Cell constant correction values are given to allow quick determination of concrete resistivity.