The macro-models method is one of the main categories for modeling infills based on the equivalent strut method. The basic parameter of these struts is their equivalent width, which affects the stiffness and strength. This paper presents a general review of several expressions proposed by researchers to calculate this equivalent width. The comparative study of different expressions shows that the Paulay and Priestley equation is the most suitable choice for calculating the diagonal equivalent strut width, due to its simplicity and because it gives an approximate average value among those studied in this paper. Consequently, the model will be used in our further study for analysis of RC infilled frames.