Gas is commonly used in measuring the permeability of low-permeability media such as coals and shales. It is well established that gas permeability depends on the pressure of the experiment, and generally yields higher values than when liquids are used. While Klinkenberg (1941) proposed a rather accurate equation to account for the slip effect, recent studies on low-permeability media have shown deviations from it. These deviations are more significant in low-permeability media. This paper presents a new analytical approach to predicting and explaining the gas slip effect. It proposes a new equation based on Klinkenberg's theoretical work and findings of Kundt and Warburg (1875), which expands the applicability of Klinkenberg's slip theory to low-permeability porous media while keeping the simplicity of the original equation. The proposed equation is validated by the experimental results of different authors. This paper further discusses the new equation's constants and the range of applicability of the theory. The new equation successfully predicts the deviations from Klinkenberg's equation in low-permeability media and reasonably matches the experimental results. (C) 2013 Elsevier B.V. All rights reserved.