From previous engineering projects it turned out that it is difficult to determine the transmitted wave height of floating breakwaters without performing physical models or using numerical models. In this thesis it is investigated which steps should be taken during the preliminary design process in order to predict the effectiveness (wave transmission) of floating breakwaters more accurately. Existing formulas which can be used to determine the transmission coefficient of floating breakwaters are compared with data of physical models. A numerical model (AQWA) is used to generate additional data for the areas of interest where physical model data is missing. Based on physical and numerical model data conclusions are drawn regarding to the applicability of the existing formulas for wave transmission. The final product of this thesis is a flowchart which can be used as a design tool during the preliminary design stage in order to predict the wave transmission of floating breakwaters more accurately.