A new method of calculating waveguide dispersion parameters is presented by combining results of Ref [3] with the Gaussian approximation. The influence of two usual Gaussian approximation methods on the dispersion calculation uses this method. A new definition of the spot size is introduced, and simple analytic formulas for the dispersion parameters of single-mode a optical fibers are given in terms of this new definition. The results calculated from these formulas are in good agreement with the numerical results.