A geometrical model of plain knitted structures is discussed in depth to understand the yarn path in a knitted loop. A theoretical model has been created to predict the porosity of a knitted structure depending on the geometrical parameters, such as course spacing, wale spacing, stitch length, fabric thickness, count of yarn and fibre density. Polyester and nylon plain knitted fabrics were produced to different tightness, and porosity was determined by measuring the weight. The validity of the model was confirmed by experimental results, using different plain knit fabrics. The porosity of a knitted structure can be changed by reducing the yarn thickness and the stitch length; however this would influence the courses and wales per unit length in the structure.