In this article we study the 3D porosity of plain weft knitted fabric while is subjected to extension under different uniaxial extension in course direction. There are a few models for 3D porosity investigation of weft knitted fabrics such as Benltoufa model and Karaguzel model as theoretical models and empirical Guidoin model. To investigate the accuracy of these models, plain knitted fabrics were produced over a different range of knitting stiffness. Thereafter, the estimated porosities of the fabrics using the theoretical models were compared with the experimental values which are obtained from Guidoin empirical model. It was concluded that Karaguzel model shows reliable and trustful result while, Benltoufa model shows huge differences in comparison with Guidoin model. In order to reduce these differences, an improved model which is named as i-model is introduced.