Local geometric imperfections on pipes under external pressure can lead to local buckling which under the “right” conditions can cause the initiation of a propagating buckle. This buckle, driven by the pressure, propagates up and down the pipe flattening it. The lowest pressure which can sustain a propagating buckle is called the propagation pressure and is a characteristic pressure of the pipe. Experiments have shown this pressure to be dependent on the post yield characteristics of the pipe material behavior, as well as the diameter-to-thickness ratio of the pipe. The collapse mechanism has been modeled by the collapse of a ring under external pressure. By identifying the lowest postbuckling pressure of the ring with the propagation pressure an empirical expression for the latter quantity has been established.