Most damaged pipelines are structurally safe, but due to hydraulic requirements they require renovations. One solution is to apply an internal polymer liner to the damaged pipeline. The critical external pressure (due to ground water pressure) at which the liner may buckle has been the subject of many studies. The most conservative choice of buckling pressure for liners is that for the case of no external support. Besides being uneconomic with respect to material quantities, it also adds more difficulty to the installation of the thick liner into the pipeline. New design tools, useful for design engineers, are extracted from a parametric study performed earlier by the author to evaluate the critical pressure of liners. The new design tools are proposed as an alternative to the available design equations and empirical correction factors recommended in the ASTM specifications.