The calibration and in particular the linearity of a three‐plate capacitance displacement transducer of range 1 mm and sensitivity 0.1 nm is measured with reference to a He–Ne laser standard using a Fabry–Perot etalon. The magnitude of the nonlinearity over the operational range is compared with estimates obtained from calculations of fringing fields. The most important contribution to nonlinearity is found to be inaccuracy in machining the capacitor plates, but this can readily be restricted to less than 1 nm over a central range of 0.3 mm (i.e., 3 parts in 106).