how to calculate the RMS value of PWM inverter voltages

I am making a single phase inverter with pwm frequency of 7.8khz from a 24 volt battery and variable modulation index which is then step up using a 24/225 v transformer. the output of the transformer is fairly sinusoidal (filtered by leakage inductance) while that of the H-bridge is square wave pwm type(low voltage side).
how do i measure the rms values for both these signals? according do the rms voltmeter readings both values are not in correspondence to the transformer ratio.


in the attached figure for modulation index 0.8
rms meter output (low side) = 16.8 volts
rms meter otput (high side) = 117 volts