California bearing ratio (CBR) is an important property used to express the quality and strength of the unbound granular materials. Moreover, it is one of the material inputs for the empirical pavement design procedure. CBR is also conducted on the unbound materials for the quality control/quality assurance during construction. Because of its importance, this paper presents a mathematical approach to the problem of estimating the strength of local sand-aggregate material usually obtained from riverbed and banks and used as subbase layer for road construction. Based on simple and easy to determine physical parameters such as particles size distribution, modified proctor dry density (MDD), and moisture content, such estimation could be determined. The study can help in understanding the nature and the properties of sand-aggregate mixture and provide a reasonable and simple statistical method for assessing the strength from simple physical tests, and assist in considerable reduction of the bulk testing of samples on a project, thus affect economy, choice of material and quality control. The model obtained is capable to explain 73% of the variation in log CBR predicted from gradation and density parameters.