dBm is defined as power ratio in decibel (dB) referenced to one milliwatt (mW). It is an abbreviation for dB with respect to 1 mW and the "m" in dBm stands for milliwatt.

Calculations:

Power level	P(dBm) ref 1 mW
Voltage	V(RMS)
Watt	P(watts)
Impedance	Z Ω

Formulas:

Here is an Excel spreadsheet containing the calculations:
dBm to Voltage to watts
and the conversion formulas here:
EMC Related Formulas