Internal diameters of a long, small bore metal tube were calculated from the measured capacitances between it and a cylindrical coaxial probe of known characteristics. The tube and the probe were used as the electrodes of a three‐lead capacitor, measurable with high accuracy in a transformer ratio arm bridge. For tubes of 0.1 cm bore or less, average diameters as a function of position can be determined by this method with an over‐all uncertainty of ±6×10−5cm over lengths of 1 m. The method can readily be extended to larger bores and greater lengths.