NRF24L01+ Power Amplifier -dBm options to milliWatt
The classic formula for converting power between dBm and mW is

$$ P[dBm]=10log_{10} \frac{P[mW]} {1mW} $$

To convert in the other direction, you use formula:
$$ P[mW]=10^{\frac{P[dBm]}{10}}$$

Few handy things to know is that for every 3 dB, power increases around two times and for every 10 dB, power increases 10 time, so for example 10 mW is 10 dBm, 100 mW is 20 dBm, 1W is 30 dBm = 1 dBW and so on.

Also keep in mind that you can add togetherunits in dB and dBm, so if you have output power of say 0 dBm and connect that to a 10 dB amplifier, you'd get 0 dBm + 10 dB = 10 dBm output power.