Soft actuators are used for automation in the food industry and medical applications due to their compliance and flexibility. However, modeling such actuators has been a challenging task owing to its softness and continuous nature. In this study, we focus on soft bellows actuator and propose a simple empirical model for estimating its output force. The model includes the parameters of geometry and material, which can be used to facilitate the actuator design and control. To validate the proposed empirical model, we first conducted finite element (FE) simulations of the bellows actuator with different design parameters and then performed experiments on actual bellows actuator which was fabricated using a 3D printer and TPU material. Both simulation and experimental results show that the proposed empirical model can accurately estimate the output force of soft bellows actuator. The model is expected to effectively facilitate the design and control of bellows actuator.