This tool converts from a dBm power value to Vpeak-to-peak. It also calculates the RMS voltage.
The default impedance (Z) is 50 ohm, although you can change that to any value.
Vpeak-to-peak = 2*SQRT((2*Z)/1000)*10(PdBm/20)
1 dBm into a 50 ohm impedance translates to Vpk-to-pk of 0.71 Volt.
The same power into half the impedance value gives a peak to peak voltage of 0.51 V.