This paper presents a model that can be used to compute the plasticity index of inorganic soil using the correlation between the liquid limit and the plasticity index. The model was developed using British fall cone Atterberg limit test results using 120 inorganic soils of widely varying plasticity, including commercial bentonite, and validated using 136 Atterberg limits test results from the literature.Analysis of the Atterberg limits data of widely varying plasticity characteristics and geological origin indicates that the plasticity index of most inorganic soil follows a well-defined trajectory in the space of liquid limit and plasticity index, which can be modeled mathematically, suggesting that each liquid limit is associated with a unique value of the plasticity index.