Power
is commonly expressed in decibel values in order to make adjustments due to loses and gains. The logarithmic
nature of decibel units translates the multiplication and division associated with gains and losses into addition
and subtraction. Once you get used to it, you will never want to return to watts. Units of dBm are decibels
relative to 1 mW of power, hence, 0 dBm 1 mW. 1/100 mW is -20 dBm and 100 mW is +20 dBm. Powers less than 1 mW are
always negative dBm values, and powers greater than 1 mW are always positive. See the graph below.

Here are the formulas:

P(mW) = 10^{[P(dBm)/10]}

P(dBm)=10*log[P(mW)]

Here are the formulas:

P(mW) = 10

P(dBm)=10*log[P(mW)]