We introduce a novel analytical approach to the evaluation of harmonic content in multi-level inverters. An analytical formula for the phase voltage total harmonic distortion (THD) of any multi-level pulse-width modulation (PWM) signal is introduced. As opposed to conventional THD expression where full spectral knowledge of the signal is required, the proposed formula depends only on fundamental amplitude, inverter's voltage level and switching angles. Hence, THD derivation may be significantly simplified. The proposed formula may be applied to evaluate THD of the multi-level output signal. It may be applied to make intelligent decisions as for the choice of various output signal characteristics such as switching frequency, switching angles placement, etc. THD formula can also serve as a cost function to minimize output signal THD while obtaining the desired control over signal's fundamental component and lower-order intrinsic harmonics. This way, in cases where multiple solutions exist (e.g., SHE), optimal solution in terms of THD may be obtained.