Electromagnetic coupling electrode is the most popular pickup to monitor beam position of an accelerator.The output signals of BPM pickup carry not only beam position information but also beam bunch charge information.Based on a study to have a better knowledge of the relationship between sum signal of electrodes and beam charge using finite element method

we developed a Matlab software package to calculate bunch charge calibration factor for different type of BPM pickup.Simulation for the storage ring BPM pickup of SSRF showed that bunch charge calibration factor variation is less than ±0.1% within 2.5 mm radius of pickup center.