A microwave paint thickness sensor includes a single cylindrical cavity, a microwave source, and a signal detector. The cylindrical cavity is open at one end, the open end having a choke joint for interfacing with a painted surface. The cylindrical cavity is designed so that the electronic field is normal to the painted surface. In a preferred embodiment, this is accomplished by providing an optimally designed TM011 mode cavity. In this configuration, the resonant frequency of the cavity is linearly related to the inverse of the paint thickness. In accordance with one aspect of the present invention, the resonant cavity is optimally sized to resonate at a frequency where the sensor footprint can be minimized. Thus with the use of the choke joint, the small sensor interface area of the present invention may easily be applied to a curved surface.