The resolution of a synthetic aperture radar (SAR) system depends not only on the operating frequency range of a radar but also on the geometry. A radar system operating at MHz frequencies facilitates SAR resolution to the meter (m) level with a linear aperture. With the same radar system, the realization of the circular aperture can further enhance the SAR resolutions to the submeter level. For GHz SAR systems, the realization of circular apertures helps to reduce the resolution from the centimeter (cm) level to the millimeter (mm) level. In this paper, a resolution equation for SAR systems synthesizing circular aperture (CSAR) is introduced. The equation is derived from the backprojection integral for CSAR, its Fourier transform, and the method of stationary phase. The accuracy of the derived equation is enhanced with a correcting factor that is numerically calculated with a cubic interpolation. Therefore, the equation can provide a more accurate, analytical, and practical estimate of the spatial resolution that can be reached with a CSAR system than the ones available in the literature. The resolution equation introduced in this paper is verified with simulations of a MHz SAR system and a GHz SAR system. The equation is further verified by an experiment with a THz inverse SAR (ISAR) system. The radar utilized in the ISAR experiment is a D-band radar system mounted on an antenna positioner that supports circular movement.