Antenna or Coax          Watts       100 Watts input
Gain in dB                Multiplier         ERP        
      1                       1.2              120 
      2                       1.6              160 
     3                       2.1              210
     4                       2.5              250
     5                       3.2              320
     6                       4.0              400
     7                       5.1              510
     8                       6.3              630
     9                       8.0              800
    10                     10.2            1020
    11                     12.6            1260
    12                     15.9            1590
    13                     20.0            2000
    14                     25.1            2510
    15                     31.6            3160
    16                     39.9            3990
    17                     50.2            5020
    18                     63.3            6330
    19                     79.5            7950
    20                   100.0          10000

Example to Calculate ERP:
If you transmit 100 watts into a length of coax that has 2 dB of loss, and an antenna with 5 dB of gain, subtract the loss from the gain and you are left with 3 dB of gain.  From the chart above, 3dB of gain at 100 watts in, equals an Effective Radiated Power of 210 watts.

Using this table, you will notice that with every 3dB increase, your are doubling the ERP!

by: AI4JI