EIRP
The effective isotropic radiated power (EIRP) is that power radiated from an isotropic dipole and can be more than the real TX power for a high gain antenna. Of cause the the gain is relative NOT real.
The EIRP Calculator
How far can you propagate?
Given the transmitter power, connecting cable loss and the antenna gain we can calculate how far the radio waves will reach. The earth radius is ignored.