An accurate procedure for the determination of second harmonic generation (SHG) efficiency from powder crystal monolayers is proposed. The method is based on the analysis of the dependence of the SHG output on the powder particle size. Unlike with conventional powder methods, the amount of material required is very small, and the scattering of the primary beam by the powder particles is negligible, allowing for an accurate analysis of the collected data. The experimental results demonstrate that the proposed method can provide reliable values for the nonlinear optical coefficients, particularly in the case of phase-matchable uniaxial materials, for which simple analytical expressions can be derived.