The relationship between operating pressures, pipe strength, pipe wall thickness, and pipe diameter is defined by the equation called the pipe strength or hoop stress equation. Each type of pipe has a specified minimum yield strength (SMYS), which is determined through testing by the manufacturer. This equation can be used to determine maximum allowable operating pressure (MAOP) of a pipe given a known operating stress and known wall thickness and diameter, or it can be used to determine the stress created by a known pressure to determine whether a proposed configuration of pipe will have a sufficient operating stress level to be used.
In some cases, additional factors may be introduced into the pipe strength equation such as temperature derating or combined stresses (such as pressure plus external loading from traffic), but in most cases, the equation applies as shown: