To design a structure against the earthquake's lateral force, we need to use a dynamic time-history analysis, which is generally cost-effective and time-consuming. Inevitably, at least in the early stages of the design of the structure, spectral analysis can be used, which requires the presence of a spectrum corresponding to the site in accordance with the regulations and the period of the original or natural frequency of the structure. To determine the periodicity of a structure that has not been designed so far, according to the current method, we use the empirical relationships in the regulations. In this paper, using theoretical relations as the basis for completing existing empirical relationships and using empirical data, a relationship is proposed to determine the period of the main structure of the structure. The structure of the above relationship is based on theoretical relations and the coefficients of the relationship are derived using a number of actual data derived from the articles.