A theoretical analysis of the influence of the substrate on the gauge factor of polymer thick-film (PTF) resistors is presented. The main advantage of PTFs is the wide range of substrates that can be used, including flexible polymers such as Mylar polyester. Nevertheless, polymer substrates are not always the optimum PTF carrier. It will be shown that their high Poisson's ratio and low Young's modulus may actually degrade the use of PTFs as strain gauges. New gauge factors are proposed, the planar GF_xy and the hydrostatic GF_h, which are favoured by polymer substrates and have high values compared to the longitudinal gauge factor GF_L.