In this project propose, analytic expression of output voltage total harmonic distortion (THD) in staircase modulated single phase multilevel inverters (MLI) is derived and verified. The revealed non-recursive formulae is expressed as a simple function of all possible switching angles, valid for generalized MLI irrespectively of the topology and number of voltage levels (odd or even). The proposed expression may be utilized for accurate calculation of THD values, eliminating the risk of underestimation associated with typical frequency based (numerical) approaches adopted by modern computational software packages.