A method for calculating the coupling inductance of active power filters (APFs) is proposed in this paper. The current ripple is defined and its relationship with the coupling inductance value is deduced based on error analyses. Hence, the lower boundary of the inductor value could be determined from the current ripple limit. A method to estimate the upper boundary of the inductor value is also proposed based on the capability to track compensating current reference. It is suggested to select the coupling inductor value close to the lower boundary for the APF. The upper boundary is used to check if any further improvement is needed for getting the balance between tracking speed and suppressing current ripple. The converter topology and modulation signal scheme are also considered in selecting the coupling inductor. Simulation and experimental results are provided to show the validity of the proposed method. Comparisons with previous methods are also given. The proposed method may also work for other inductor-coupled system, such as STATCOM.