Adding Power Levels...

Adding two power levels is easy when using Watts or milliWatts, but adding power levels expressed in dB is not as simple!  Whereas 1mW + 2mW = 3mW, 0dBm + 3dBm (the equivalents of 1mW and 2mW) DOES NOT = 3dBm!  0dBm + 3dBm = 4.77dBm!  Also, try adding 3mW + 2dBm (3mW + 1.58mW = 4.58mW or 6.61dBm)!! The calculator below makes adding two power levels in mW, W, dBm or dBW easy; one can even "mix 'n' match"!!
NOTE:  The units of power MUST be selected before the calculator will work.

(1) Select units of 1st power to be entered:
 mW       W      dBm      dBW

(2) Highlight "1" in left-hand field and enter 1st power level:

 =   mW &   dBm
(3) Select units of 2nd power to be entered:
 mW      W      dBm      dBW
(4) Highlight "0" in left-hand field and enter 2nd power level:
 =  mW &   dBm

RESULT:    mW    W
   dBm    dBW

Note that the magnitude of dBW for a given power level is always "30" less than the magnitude of dBm. This is due to the relationship between Watts and milliWatts.
dBW = 10*logP where P = power is in Watts.
dBm=10*logP where P = power in milliWatts.
Example:  Using 1W = 1000mW (1000 = 103),                     
dBW = 10*log1 = 10*0 = 0 while dBm = 10*log103 = 10*3 = 30
An ND2X javascript calculator