The following chart may be used as a starting point for converting Watts to dBm and dBm to Watts.
Note: The decibel is a logarithmic unit. DBm or decibels in milliwatts is referenced to 0 dBm equalling 1 milliwatt.
Memorize the dBm, mW & W columns and the 3 rules that follow and you’ll be able to do rough power calculations in your head in no time at all!!!