Abstract: In computer control of machine tools there is a need to measure the diameters of cylindrical workpieces automatically and preferably without touching the workpiece. For this purpose an inductive proximity sensor is mounted for circular or linear movement along a path adjacent to a cylindrical workpiece and in a plane normal to a radius of the workpiece. A discrete fast Fourier transform (FFT) is carried out on the output of the sensor as it moves along the path and the amplitude of the second ac component of the FFT is an indication of the workpiece diameter. By calibrating with workpieces of different diameters, the said amplitude can be used as a measure of workpiece diameter.