In the wall of an oval pipe, additional lateral forces and moments of flexion occur under internal pressure loads. Thereby, the bending stresses reach the manifold of normal stresses in certain cross-sections and consequently are of decisive importance for the dimensioning of the pipe wall. The traditional calculation method for the moments of flexion provides only approximate values. The application of the partially plastic equation of flexion-introduced in this essay instead of the elastic one-provides more precise values for the wanted moments of flexion in the checked cross-sections. The exploitation of the material to a fully plastic condition of flexion leads moreover to a smaller wall thickness of the flat-oval pipe.