The accurate estimation of the performance change according to variations in stack length of motors is important in related industries. A permanent magnet (PM) motor usually represents a characteristic in which its back electromotive force (EMF) constant (K e ) is linearly proportional to stack length. It is an enormous advantage in designing and estimating the performance of PM motors. However, the linearity cannot be held in spoke-type ferrite magnet synchronous motors (SFMSM) due to the axial leakage magnetic flux that is irrelevant to the stack length. Thus, it is possible to accurately analyze the performance of SFMSM using a 3-D finite-element analysis (FEA) method but this method is very inefficient. In this paper, a new relationship formula that can consider the nonlinearity between stack length and back EMF constant in SFMSM is proposed. This formula can estimate the back EMF constant in a new stack length based on the results of 2-D-FEA and 3-D-FEA in a specific stack length. Finally, the validity of this formula is verified through analyzing its accuracy and testing it according to changes in motor parameters.