The air permeability of a number of commercial fibrous carbon materials: soft non-woven felts, rigidised felts and rigid boards, based on either PAN- or Rayon-derived fibres presenting various diameters, graphitised or not, and consolidated by different methods, was measured and investigated. Consistent behaviours were found within families of closely related materials, but the diversity of porous structures prevented any model, including the very popular Tomadakis-Sotirchos equation, to fit all results. The Archie's coefficient and the tortuosity factor for viscous flow were thus calculated. Not only all data were perfectly aligned on one single master curve, but the analysis was extended to many other fibrous materials and the same master curve was found to be relevant. The Archie's coefficient thus appears to be an intrinsic property, purely defined by the material geometry, as it does not depend on the 1D, 2D or 3D type of flow. A fitting equation was proposed, encompassing all fibrous materials in very broad ranges of porosities and porous structures.