In this paper, a method based on an iterative 3D finite element modelling is developed to determine with high precision the demagnetization factor for various geometries leading to the correct material behaviour law. This is further used to accurately correct the magnetization versus applied magnetic field obtained from open circuit measurement method such as vibrating sample magnetometer. This method is seen to be more accurate than existing methods and agrees well with the well-known formulas in the case of simple geometries with a relative error smaller than 0.2 %.