This tool converts Power in dBm to Volt Root-mean-square or VRMS
The default impedance (Z) is 50 ohm, although you can change that to any value.
VRMS = SQRT(Z/1000)*10(PdBm/20)
1 dBm into a 50 ohm impedance translates to an RMS Voltage of 251 millivolt.
The same power into half the impedance value gives 177 mV.