Consistency limits and soil plasticity are the basic parameters that represent the physical properties of fine-grained soils and provide important information about their engineering properties, such as the compressibility and shear strength. These parameters are the standard input for all soil investigation programs, so their correct definition and usage in the design have great importance. The liquid limit value, which is one of the parameters that determine the consistency properties of the soils, can be obtained by standardized mechanized experiments that minimize operator effects. Meanwhile, the plastic limit is calculated by an experiment that is not standardized and can be affected by the operator's experience. In this case, the important parameters that affect the soil behavior are obtained in a wide range of values, and the reliability of the results is debatable. Thus, this paper aims to develop a new correlation, where the plasticity index can be directly calculated based on the liquid limit obtained from standardized experiments. Thus, the effect of the operator on the plastic limit test is eliminated. The effect of mineralogy is also considered in the newly developed correlation in addition to previous studies in the literature. A comprehensive experimental program was applied to 301 high-plastic clay samples, which were taken from the Bakirkoy region of Istanbul city (Turkey), and the results were used to obtain liquid limit values using the Casagrande cup method.