This paper presents a new formula to estimate the ultimate lateral force of unreinforced masonry structures. The ratio of the wall's height to the wall's width is used to predict the ultimate lateral load of the wall. The coefficient is determined by the numerical implementation of an interface model to simulate the behavior of mortar joints in masonry walls. The numerical predictions are compared with the FEMA guidelines and the experimental data. The comparisons show that the loads that are predicted by the proposed formula have a lower error percentage than the FEMA guidelines. Thus, the proposed formula can be used to analyze unreinforced masonry structures.