The study of flow and transport in unsaturated soils requires a good knowledge of the soil properties. When advective gas phase transport is of interest, air permeability is the key parameter of soil characterization. Although an air permeability test can easily be done in a laboratory, the reported values are frequently found to be not very representative of the air permeability in situ. Air permeability determined by a field test is useful both in the study of gas flow through unsaturated soils and in the prediction of the saturated hydraulic conductivity. This paper presents analytical solutions for determining the vertical air permeability of unsaturated soils using observed air pressure data at the land surface and at depth in the vadose zone. One solution is developed for a single-layer soil, and the other is developed for layered soils. Example calculations using field data have demonstrated the significant potential of the solution applications in field tests.