Infinite slope (IS) method is the simplest limit equilibrium method for slope stability analysis. It gives reliable results for slides where the longitudinal dimension prevails on the depth of the landslide. Usually results are conservative since ignoring the effects of the strength along lateral bounds. Here, starting from the assumption of considering the effects of the shear strength along lateral bounds by a rectangular cross section, a new expression of the safety factor is investigated, based first on an elliptical and then on a parabolic cross section of the landslide mass. The safety factor evaluated in this way can be quite different from those returned by the classic formula of IS model, in particular when the width of the landslide is narrow with respect to its depth and the ratio between the width and the depth of the landslide is lower than 5. An interesting implication of the proposed model is that if cohesion differs from zero, there is a ‘critical depth’, where the safety factor has a minimum value. Copyright © 2013 John Wiley & Sons, Ltd.