The difficulty of making accurate, repeatable, sub-nanometer displacement measurements has limited the progress of nanotechnology and surface science. Scanning probe microscopy, an important set of tools for characterizing nanoscale structures, is capable of atomic resolution imaging, but has yet to realize its full, metrological potential. This work evaluates the feasibility of using microwave cavities to address this need. Accurate and stable RF frequency references have become ubiquitous and are an attractive option for realizing traceable distance measurements through the resonant frequency of microwave cavities operating in TEM modes. A method of measuring the resonant frequency of such cavities capable of sensing picometer displacements is developed. The concept is demonstrated with a variable-length, 10 GHz coaxial cavity, and proves to have a resolution of 60 fm Hz^-1/2, and a range of 10 μm. Independent measurements with an interferometer verify that the device is capable of displacement measurements accurate to 1% without external calibration, and with non-linearity <5×10^-4 of the measured range. Appropriate mechanical design can extend the range and improve the accuracy. Incorporating this system into scanning probe microscopes would allow them to measure sub-atomic distances confidently.