This study investigates the sensing performance of a conductometric sensor for NH3 gas based on Ca-doped ZnO nanoparticles with different calcium concentrations (0, 1, and 3 at%) synthesized using the sol–gel process under supercritical dry conditions of ethanol. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectroscopy. The results show that pure and Ca-doped ZnO are polycrystalline and well crystallized in the hexagonal wurtzite structure. The average crystallite size increased with the addition of Ca, and the gas sensing performances were clearly improved with the 1% Ca-doped ZnO sensor. The 1% Ca-doped ZnO sensor exhibited a gas response of 33 for 4000 ppm of NH3 at T = 300 °C with good selectivity compared to other gases such as CO, CO2, and NO2. The response and recovery times were 5 s and 221 s, respectively. The study demonstrates the potential application of Ca-doped ZnO as a sensor material for ammonia detection.This study investigates the sensing performance of a conductometric sensor for NH3 gas based on Ca-doped ZnO nanoparticles with different calcium concentrations (0, 1, and 3 at%) synthesized using the sol–gel process under supercritical dry conditions of ethanol. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectroscopy. The results show that pure and Ca-doped ZnO are polycrystalline and well crystallized in the hexagonal wurtzite structure. The average crystallite size increased with the addition of Ca, and the gas sensing performances were clearly improved with the 1% Ca-doped ZnO sensor. The 1% Ca-doped ZnO sensor exhibited a gas response of 33 for 4000 ppm of NH3 at T = 300 °C with good selectivity compared to other gases such as CO, CO2, and NO2. The response and recovery times were 5 s and 221 s, respectively. The study demonstrates the potential application of Ca-doped ZnO as a sensor material for ammonia detection.