Generally power is expressed in watts. The gain is the ratio of the output power to the input power. Gain = (output power)/(input power) if the gain is expressed in terms of log(base10) it is known as Decibel power gain Decibel power gain = Log10(Gain)dB if 1milliwatt power is taken as reference then Decibel power gain = Log10(Output power/1 milliwatt)dBm