The paper studies the stability of unsupported tunnel faces by analyzing the results of a large number of 3D numerical analyses of tunnel faces, in various ground conditions and overburden depths. The analyses calculate the average face extrusion (Uh) by averaging the axial displacement over the tunnel face. Limiting face stability occurs when the average face extrusion becomes very large and algorithmic convergence becomes problematic. Using the results of the analyses, a dimensionless “face stability parameter” is defined, which depends on a suitable combination of ground strength, overburden depth and tunnel width. The face stability parameter correlates very well with many critical tunnel face parameters, like the safety factor of the tunnel against face instability, the average face extrusion, the radial convergence of the tunnel wall at the excavation face, the volume loss and the deconfinement ratio at the tunnel face. Thus, semi-empirical formulae are proposed for the calculation of these parameters in terms of the face stability parameter. Since the face stability parameter can be easily calculated from basic tunnel and ground parameters, the above critical tunnel parameters can be calculated, and conclusions can be drawn about tunnel face stability, volume loss and the deconfinement ratio at the excavation face which can be useful in preliminary tunnel designs.