Display omitted •Amounts of Pt and Pd nanoparticles on SnO2 nanowires were optimized for NO2 gas sensing improvement.•The NO2 gas responses showed a bell-shaped dependency on the amount of Pt and Pd nanoparticles.•The approximation model can be used to find the optimum amount of the metal nanoparticles. The functionalization of noble metal nanoparticles (NPs) is a highly efficient method for increasing the sensing performance of metal oxide nanowire (NWs) gas sensors. Despite the well-established strategy, the level of the optimal functionalization for obtaining the maximum sensing response has rarely been reported. Herein, the surfaces of SnO2 NWs were functionalized with Pt and Pd NPs, and the gas sensing characteristics were then investigated using NO2 as an example gas. The sensing responses obtained at the optimal temperature showed a bell-shaped dependency on the proportion of surface coverage by Pd and Pt NPs. The sensing mechanism is explained and the results were also fitted to a simple theoretical model based on modulation in the conduction channel via the chemical and electronic sensitization on NWs. This study provides guidelines on the amount of metal NPs for achieving the optimal sensing responses in metal oxide NWs gas sensors.