This paper presents an improved analytical calculation model for unstiffened elements with nonlinear stress-strain relationships (e.g. aluminium and stainless steel) subjected to uniform in-plane compression. The model takes material nonlinearity into account by means of the Ramberg-Osgood constitutive law. It is based on the results of a numerical parametric study of the load-carrying behaviour of unstiffened elements. A comparative study shows good agreement between test results and analytically determined strengths according to the new calculation model.