A novel and universal model to calculate the effective width of edge-less FETs has been proposed in this paper to solve the problem that the calculations of effective width in commercial SPICE model does not fit annular gate devices.This model is based on channel electric field distribution in annular gate and can be used for most shapes of annular gate devices.Corrections of SPICE model parameters have been proposed to make the model more accurate.Verifications have also been done with different gate geometries in different ways such as TCAD (Technology Computer Aided Design) and so on.Simulations results show a good accuracy of the proposed model.