Code design of Unreinforced Masonry (URM) buildings is based on elastic analysis, which requires as input parameter the effective stiffness of URM walls. Current approaches estimate the effective stiffness as fixed ratio of the gross sectional stiffness but comparisons with experimental results have shown that this does not yield satisfactory predictions. In this paper, a recently developed analytical model for the force-displacement response of URM walls is used to investigate the effective stiffness. First, the key features of this model are summarised. In further course, the model is used to predict the effective stiffness of full-scale tests on modern unreinforced clay brick masonry walls and the results are compared to the provisions of Eurocode 8. Concluding, the model is used to investigate the sensitivity of the effec-tive stiffness to the wall geometry and axial loading.