Flexible electronics have attracted wide attention for a variety of applications in the last few years; some based on components such as diodes and transistors have been reported earlier. This paper reports an optimized design of a flexible film bulk acoustic resonator suitable as a sensor for toluene gas detection with polyethylene terephthalate as flexible substrate. Aluminium nitride, lead zirconate titanate, and zinc oxide were chosen as the piezoelectric materials, and a polydimethylsiloxane (PDMS) coated electrode was placed on top to perform as a functional layer for sensing. Finite element modeling was used for the optimization of PDMS flexible film. The paper also reports the effect of different piezoelectric materials used, Bragg reflector stages and effect of PDMS layer thickness on the performance of the sensor. The optimal sensor configuration was obtained using Taguchi DoE and ANOVA techniques. For the optimized structure, simulated values of coupling coefficient, quality factor and figure of merit are 0.237574 (or 23.7574%), 991 and ~ 235, respectively. The sensor exposure to toluene in the concentrations from 0 to 500 ppm resulted in down shifting of its resonant frequency. Sensitivity of 12 kHz/ppm has been observed and reported.