Determining the optimum weight of the armour blocks is the most important issue in designing the break water. Existing empirical formula, such as those of Hudson (1958) and Van der Meer (1988), depict considerable scatter between the predicted stability number and the measurements. This scatter imposes higher safety factor and increases the construction costs (Kim and Park, 2005). Numerous researchers put efforts in this issue to increase the accuracy of the prediction by using different mathematical models (i.e., Mase et al., 1995; Kim and Park, 2005; Erdik, 2009; Etemad-Shahidi and Bonakdar, 2009; Etemad-Shahidi and Bali, 2012). However, the proper definition for the permeability parameter of the rubble-mound coastal structures is yet missing. In this paper, a semi-empirical model for permeability parameter is introduced and improvements in designing of the rubble-mound breakwater is presented.