Although several complex computer programmes available in the literature provide detailed information on both the overall characteristics and internal magnetic and electric fields of FCGs, they usually involve considerable computing time. However, the present interest in the use of FCGs as the energy source for high power microwave and other equipment brings about the need for a fast code for use in the preliminary stages of a system design. The paper describes a programme that is much faster than the detailed programmes, but nevertheless provides an accurate prediction of the terminal characteristics. A step-by-step procedure is used to calculate the different inductance variations, and both skin and proximity effects are taken into account in determining the resistances. Theoretical predictions are compared with available experimental data for several familiar generators and the results are analysed. The most important parameters for a high-energy gain design as suggested by the fast code are highlighted.