The article presents a method for calculating the mass load for determining the displacement of passenger trimaran vessels at the initial stages of design. The state of this issue has not been sufficiently studied to date, since there is no methodology that allows calculating the displacement of a trimaran vessel. The field of research is the design of vessels, in particular the design of passenger trimaran vessels, and the purpose is to determine the components of the mass load to determine the total displacement of the trimaran vessel. The equations of masses for trimaran passenger ships and statistical dependencies are given, as well as analytical formulas for finding the components of this equation. On the basis of a comparative analysis of prototype vessels and projected trimaran vessels, satisfactory results were obtained, which showed that the proposed methodology can be used to calculate the displacement of vessels of this type at the initial stages of design.