In the process of carbothermic reaction of SiO₂ + mC, key reactions appear on the surfaces of both SiO₂ and C grains. However, the values of these surfaces are not known. Assuming the simplest case, quartzite and carbon grains are spheres, total surfaces of reaction were calculated for grains of carbon and quartzite respectively. This enabled to determine the rate of weight loss referred to the unit area of C and SiO₂.