The normal value with which we refer to the voltage is called the rms voltage. It is the root mean square of the instantaneous voltage supply
V rms = Sq. root (v1+v2+v3+v4.....vn)/n
Where v1, v2 , v3 and vn are the instantaneous voltages.
The relation between the rms voltage and the peak voltage is as follows.
Vrms = 0.707 x V peak