The turning radius of the vehicle is determined based on the coordinates of the center of gravity. When the vehicle is moving at different velocities and driving at different steering angles, the value of the turning radius is also different. This change is nonlinear and continuous and determined by simulation or experimentation process. Previous studies have not established the formula to calculate the value of a turning radius of the vehicle based on input parameters. Therefore, determining the value of a turning radius is complex and time-consuming. This research has shown the dependence between turning radius R on input parameters such as velocity v and steering angle by using a double-track dynamic vehicle model to simulate. Besides, this research also established the state function to calculate the turning radius of the vehicle at different velocities and steering angles. The state function is determined based on the rule of change of the corresponding values. Using only the determined input parameters like velocity and steering angle, the value of the turning radius can be easily calculated with high accuracy through 4 steps. The reference coefficients needed to calculate are also provided in this research. The results of the research can be applied in various computational conditions.