The report describes a new correlation for predicting gas permeabilities of undisturbed or recompacted soils from their average grain diameter (d), moisture saturation factor (m), and porosity (p). The correlation exhibits a geometric standard deviation (GSD) of only 1.27 between measured and predicted permeabilities for the Florida soils, with no residual bias, and a GSD of 1.44 for the combined Florida and previously obtained Utah data sets with a 3% negative bias. Results of a literature review of previous relations between soil properties and gas or water permeabilities show that previous exponents for particle size dependence have ranged from 1.11 to 2.05, consistent with the present value of 1.33. The work was performed in connection with the broader Florida Department of Community Affairs (DCA) effort to define radon-protective standards for sub-slab fill materials. It also has applications in EPA and other agency efforts to assess radon potentials of soils and correlate indoor radon concentrations with source strengths.