In this article a comparison between two non-linear finite element approaches for the numerical estimation of the ultimate failure load of masonry arches is presented. According to the first model, the geometry of the arch is divided into a number of unilateral contact interfaces which simulate potential cracks. Opening or sliding for some of the interfaces indicates crack initiation. The second model uses two-dimensional finite elements for the simulation of the arch. When tensile stresses appear, upon an adaptive stepwise procedure, the corresponding elements are replaced by unilateral contact elements which represent cracks. In both models the fill over the arch, that could strongly affect the collapse behaviour increasing the bridge load carrying capacity, is taken into account. Moreover, the ultimate load and the collapse mechanism have been calculated by using a path-following (load incrementation) technique. Both models are developed and applied on a real scale masonry arch; results are comparable with both the experimental collapse mechanism and the ultimate load failure.