Abstract: A coating thickness gauge is provided with a facility to store one or more zero offset values and to automatically subtract a chosen zero offset value from a measured coating thickness value. The gauge may be used to conveniently determine the thickness of coatings applied to rough surfaces using the zero offset calibration method where the gauge is provided with a key or keys. In order to prevent accidental adjustment of the calibration or any other important operational parameter of the gauge said key or keys must be depressed and held for a predetermined period of time.