Thermal and epithermal absorption of neutrons by a cylindrical control rod are computed using two-group diffusion theory and assuming a flat source distribution for the epithermal group. Hurwitz-Roe area rule concepts are modified to take into account the nonthermal absorption by the rod and are extended to include rods that are not “black.” The effect of the control rod on the neutron economy is averaged over a cylindrical cell surrounding the rod and is expressed in terms of two-group absorption cross sections. Control rod power perturbations in a reactor are calculated from the two-group flux distributions in the cell containing the centered control rod.