It’s not difficult, just always having some doubts…
Power
$latex L_{dB} = 10 log_{10} ( ) $
10 dB increase for a factor 10 increase in the ratio
3 dB = doubling
40 dB = 10000 times
Amplitude
$latex L_{dB} = 10 log_{10} ( ) = 20 log_{10} ( ) $
dBm
dBm is an absolute value obtained by a ratio with 1 mW:
$latex L_{dBm} = 10 log_{10} ( ) $
- 0 dBm = 1 mW
- 3 dBm ≈ 2 mW