Conversion from dBm to dB\$\mu\$V?
I use 0.632 volts PeakPeak for 50 ohms and one milliWatt; or 0.223 volts RMS for 50 ohms.

Makes thinking about systems a much easier task.

This comes from using one of the Watts Law forms: P = V^2/R

Given that, what is the RMS voltage for -100dBm in a 50 ohm system? (which is -130 dB Watt or -130 dBW)

What is 100 dB down from 0.223 volts RMS?

what is 0.223 / 100,000? Its 2.23 microVolts RMS.