Peridynamics is based on integro-differential equations and has a length scale parameter called horizon which gives peridynamics a non-local character. Currently, there are three main peridynamic formulations available in the literature including bond-based peridynamics, ordinary state-based peridynamics and non-ordinary state-based peridynamics. In this study, the optimum horizon size is determined for ordinary state-based peridynamics and non-ordinary state-based peridynamics formulations by using uniform and non-uniform discretisation under dynamic and static conditions. It is shown that the horizon sizes selected as optimum sizes for uniform discretisation can also be used for non-uniform discretisation without introducing significant error to the system. Moreover, a smaller horizon size can be selected for non-ordinary state-based formulation which can yield significant computational advantage. It is also shown that same horizon size can be used for both static and dynamic problems. Keywords Peridynamics • Horizon • State-based • Ordinary state-based • Non-ordinary state-based 1 Introduction Solid mechanics is an important area of engineering dealing with deformations of materials and structures under external loading conditions. Continuum mechanics has been widely used for this purpose for the last two hundred years, and there are currently different continuum mechanics formulations available in the literature with different advantages and limitations. The most common continuum mechanics formulation was developed by Cauchy, and the equation of motion of the fundamental object of continuum mechanics, i.e. "material point", is expressed in the form of a partial differential equation. Since the analytical solution of this equation is limited to particular geometries, boundary conditions and material systems, different numerical techniques including finite element method have been developed to obtain solution for numerous problems of interest. However, due to the spatial derivatives in the partial differential equations, the standard solution procedures become invalid if discontinuities such as cracks exist in the solution domain. In such cases, additional steps should be taken to get around the discontinuity problem. As an alternative approach, Silling [1] developed a new continuum mechanics formulation and named it as "peridynamics". There are several fundamental differences between peridynamics and Cauchy's continuum mechanics (CCM) formulation. First of all, the equation of motion of a material point in peridynamics is in integral form and does not contain any spatial derivatives. Therefore, it does not suffer from the discontinuity problem mentioned earlier. Moreover, peridynamics [2-20] is a non-local continuum mechanics formulation Communicated by Luca Placidi.