Practical applications of harmonic generation to determine the nonlinearity parameter Beta generally tend toward the lower drive (fundamental) frequencies. As a result, diffraction effects on Beta become more significant. Derivations of a number of different diffraction correction formulas, which are applied for a piston source and a receiver located some distance away, are in the literature. In the paper by Blackburn and Breazeale [J. Acoust. Soc. Am. 76, 1755–1760 (1984)] the correction formulas were applied to the amplitude of the fundamental frequency and the results on Beta were experimentally tested. By building on this earlier work, a diffraction correction formula that includes a factor from a numerical integration algorithm applied to the harmonically generated wave is provided. The formulation of this algorithm is discussed. Experimental results are given for measurements of Beta in AA 2024 to illustrate the agreement.