In this study, the relationship between air diffusivity and permeability in cementitious materials was investigated. First, we derived an equation to correlate air diffusivity and permeability in a straight circular tube. Then, we reviewed existing studies that measured both air diffusivity and permeability and compared reported data and calculated values to verify the applicability of the derived equation to cementitious materials. Although a correction factor was not used, the two sets of data showed good agreement quantitatively. This indicates that the derived equation can be applied to cementitious materials including concrete, and measured air diffusivity can be converted to permeability of concrete and vice versa using the derived equation.