A mathematical analysis is conducted on the experimental results of a study conducted on epoxy-based glass fiber reinforced plastics (GFRP) composites. Three different types of reinforcements were used in the form of woven fibers. Fabrics made of glass fibers, carbon fibers, and (glass-carbon) hybrid fibers were used at different volume fractions. The rule of mixtures (ROM) and the Hapin-Tsai (H-T) equations were used to analyze the experimental results. It was found that the ROM equations give approximate upper bound values for all investigated composites, for all considered volume fractions. It was also found that there is good agreement between H-T calculated values of fracture stress and those values that were determined experimentally for all volume fractions investigated. Elastic modulus values calculated according to H-T equations showed good agreement with experimental values at low reinforcement volume fractions. However, as the volume fraction increased, the experimental and calculated values diverted from each other considerably. A correlation function was calculated based on the differences between the elastic modulus values determined experimentally and those derived from H-T calculations. This function was applied to H-T values and thus, the differences between them and the experimental data were reduced.