I understand these  lectures on dB calculation properly. I learnt here four terms : 

1.dB

2.dbW

3.dBm

4.dbI

★ 1 bel = 10 decibel 

dB is actually used in power level measurement for satellite communication,telephone system, mobile system. That means signal to noise ratio level to find out value decibel system can be used.

★Power ratio = p2/p1

Here,p2 can be gain or can be lost.It depends on attenuation. But p1 power is transmitted at the transmit end and p2 power receives at the receiving end.There is no unit for power ratio.

10log10 P2/p1  ----> general formula 

10log10 Pin / Pout

★Power ratio to dB = 10log 10 (P)

dB to power ratio = 10 ^ (dB / 10) 

[ Power cannot be negative  ]

If the power unit is W in the given question , then the final result unit will be dbW.

If the power unit is dbW in the given question, then the final result unit will be W.

★ dbI 

Here,I means Isotropy. 

The source where the power is uniformly distributed to the surface that source is called isotropic source.

★ As we know, 0 dbW = 30 dBm ,,,,,,,,, so for converting dbW to dBm we have to always add 30 and  for converting dBm to dbW we have to always subtract 30. 

dbW to dBm [ add 30 ]

dBm to dbW [ subtract 30 ]

★ dbW + dbW = dbW

dbW - dbW = dB 

dbW + dB = dbW 

dbW - dB = dbW 

dBm + dBm = dBm 

dBm - dBm = dB

dBm + dB = dBm 

dBm - dB = dBm