A method and apparatus is disclosed for electrically measuring the inside diameter of a metallic pipe, such as oil or gas well casing. The transimpedance of two coaxial coils within the pipe is determined and then resolved into its inphase and quadrature components. The difference of the magnitudes of the two components is equal to an expansion equation which is a function of the inside diameter of the pipe and a factor proportional to the ratio of the permeability to the conductivity of the pipe. The inphase component of the impedance is equal to a second expansion equation which is a function of the inside diameter of the pipe and the factor proportional to the ratio of the permeability to the conductivity of the pipe. The two equations are solved simultaneously to generate signals for the pipe's inside diameter and the ratio of permeability to conductivity.