The present empirical study aims to achieve the highest cumulative yield of hemispherical solar distillers. In order to achieve this goal as well as to achieve the maximum possible benefit from the large surface area of receiving and condensing that characterizes the hemispherical solar distillers from the other traditional single-slope distillers, black gravel with different sizes were used to increase the rate of absorption of solar rays and as sensible heat storage materials, thus increasing the temperature of the basin saltwater and improving cumulative yield of hemispherical distillers. To obtain the best size of the black gravel that achieves the highest cumulative yield of hemispherical distillers, four different sizes of black gravel (4, 8, 11, 16 mm) were studied and compared to the reference hemispherical distiller under the same climatic conditions. The results presented that the cumulative yield of hemispherical distillers for utilizing the black gravel reached 5.7, 6.45, 6.9, and 7.7 kg/m² day for gravel size of 4, 8, 11, and 16 mm, respectively, compared to 4.9 kg/m² achieved by the reference distiller. The improvement in cumulative yield for utilizing the black gravel reached 16.3%, 31.6%, 40.8%, and 57.1%, respectively. Also, the improvement in daily efficiency for utilizing the black gravel reached 16.2%, 31.2%, 40.1%, and 56%, respectively, compared to reference distiller. The results concluded that the highest cumulative productivity of the hemispherical solar distiller was achieved at 16 mm black gravel size.