A simple optical microscopy method has been developed for measuring particle diameters and particle-particle gaps with a precision of a few nanometers. The method uses an inexpensive and straightforward image-processing technique to examine optical microscope images of the particles, which can be wet or dry. The technique is demonstrated on polystyrene latex and silica particles with diameters. greater than 450 nm suspended in KCl solutions, and the measured particle diameters compare to within a few percent with those measured by electron microscopy and light scattering. In addition, changes in the gap between two colloidal spheres were measured with nanometer precision, a result that will have important implications for measuring colloidal force profiles between two Brownian particles.