Processes in scheme of resonant inverter with bidirectional conductivity switches in half-bridge connection are described accenting complexity of correct calculation of processes characteristic with development of transient from cycle to cycle. Approximated calculation method based on solution of differential equations is proposed which allows rather correctly calculate extreme voltages of capacitor of resonant circuit at different parameters of circuit in steady-state resonant operation case when control switching frequency is equal to frequency of self-oscillations of circuit. Is given also more approximated calculation method based on influence on circuit a fundamental component of inverted voltage which has square shape. Comparison of obtained results for two different calculation examples with modeling results is done and results are sufficient good. Applying the last proposed calculation method it is possible find out necessary volume of capacitor for preset value of extreme voltage across capacitor plates and given parameters of investigated coil.