1.If we divide the value of the same unit, there is no more unit.

2.Formula of dB calculation 10log10(P2/P1)

3.To convert power raito from dB  the formula is 10^dB/10

4.0dBw=30dBm

5.to convert dBm from dBw  add 30 and the unit also exchange

6.To convert dBm from dBw subtract 30 and unit also exchange

7.-40dBw = -10dBm

8.And 30dBw - 10dB = 20dBw