Risk assessment and design of remediation methods at soil sites polluted with gaseous phase contaminant require an accurate description of soil-gas diffusion coefficient (D_p) which is typically governed by the variations in soil air-filled porosity (v_a). For undisturbed volcanic ash soils, recent studies have shown that a linear D_p(v_a) model, taking into account inactive air-filled pore space (threshold soil-air content, v_{a, th}), captured the D_p data across the total soil moisture range from wet to completely dry conditions. In this study, we developed a simple, easy to apply, and still accurate linear D_p(v_a) model for undisturbed volcanic ash soils. The model slope C and intercept (interpreted as v_{a, th}) were derived using the classical Buckingham (1904) D_p(v_a) power-law model, v_a^X, at two soil-water matric potentials of pF 2 (near field capacity condition) and pF 4.1 (near wilting point condition), and assuming the same value for the Buckingham exponent (X=2.3) in agreement with measured data. This linear D_p(v_a) prediction model performed better than the traditionally-used non-linear D_p(v_a) models, especially at dry soil conditions, when tested against several independent data sets from literature. Model parameter sensitivity analysis on soil compaction effects showed that a decrease in slope C and v_{a, th} due to uniaxial reduction of air-filled pore space in between aggregates markedly affects the magnitude of soil-gas diffusivity. We recommend the new D_p(v_a) model using only the soil-air contents at two soil-water matric potential conditions (field capacity and wilting point) for a rapid assessment of the entire D_p-v_a function.