ERP and EIRP
Effective radiated power is the definition of directional radio frequency power. The ERP corresponds to the power that should be radiated by a half-wavelength dipole antenna to achieve the same field strength at a given distance and direction of the transmitter. The Effective isotopic radiated power (EIRP) is the total power that would be radiated by a isotopic antenna to achieve the same field strength at a given distance and direction of the transmitter
Formulas
P is the symbol for power and is measured in watt (W).
A is the symbol for gain.