Accurate measurement of the cross-sectional area of porous fabric is quite important to determine fabric strength. Many models using yarns cross-section were built to calculate the strength of woven and knitted fabrics, and the spaces between fibres in the yarns were always considered to be part of the cross-sectional area. In our previous study, using micro-computed tomography (CT) and reconstructed three-dimensional (3D) images and based on the evaluation of the pore diameter distribution across the thickness direction, a new method was established to accurately determine the fabric ‘surface’ and fabric thickness. In this study, we used this method to calculate the cross-sectional area of fabrics and the resulting fabric stress. Using a plain weave and a flat knit fabrics showed that the cross-sectional areas in warp (or wale) and weft (or course) directions of the fabrics change regularly based on fabric structures and yarns. The fabric stress, when calculations were based on the least cross-sectional area, were greater than that calculated based on the other methods, such as the stress calculated based on the Peirce yarn model, where yarns are regarded as solid cylinders. Other research and properties related to the fabric thickness and cross-sectional areas will benefit from this accurate determination.