The air permeability of a fabric is defined as the amount of air passed over a surface under a certain pressure difference in a unit time. This value has significance with respect to the usage area. Since knitted fabrics have a loop structure, they have more pores than woven fabrics; therefore, in general, the air permeability of knitted fabrics is higher than that of woven fabrics of the same weight. An experiment to determine the air permeability is very important as it defines the properties of keeping warm, protection against the wind, breathability etc. of knitted fabrics used as clothing. In this study, it has been attempted to establish a theoretical model for the porosity and predicted air permeability of plain knitted fabrics. A theoretical model was created to predict the porosity and air permeability of a knitted structure depending on the geometrical parameters, such as the courses per cm, wales per cm, stitch length, fabric thickness, yarn count, diameter of yarn and fiber density. For this purpose, a theoretical model of porous systems based on D'Arcy's law was used, the validity of which was confirmed by experimental results using 100% cotton plain knitted fabrics produced from ring and compact yarns of different yarn number linear density and tightness.