Ronchi rulings may be used for fast, accurate measurements of Gaussian beam diameters. In this paper, we present a theoretical analysis of the Ronchi measurement method. The major theoretical result is a very simple relationship between beam diameter and the measured parameters, P max and P min. The Ronchi method is then compared with the more conventional pinhole, slit, and knife edge methods. The major advantages of the Ronchi method are discussed, and some notes on the application of the method are given.