When analyzing the propagation of optical radiation in a turbulent atmosphere, the structure function is a crucial tool. We propose to improve the method of its calculation by expressing the function analytically, via the mode coefficients of wavefront expansion. We derive formulas for the structure function on a circular aperture. Unlike the previously published analytical solution, ours correctly accounts for the entire aperture area. We compare our method to the previously published one and to numerical calculations with sufficiently fine discretization. Our test samples comprise Kolmogorov wavefronts, Zernike polynomials, and Karhunen–Loève functions corresponding to the Kolmogorov turbulence model. We show the deviation in results between our method and the others. Then, we explain the generality and other advantages of the new method. It enables an exact calculation of the structure function of a wavefront from its mode coefficients in problems of optical radiation propagation in randomly inhomogeneous media.