Use this calculator to find the max output power of a transmitter in Watt from the antenna gain (dBi) and Effective Isotropic Radiated Power (EIRP).
Watt to dBi Formula
PTx = EIRP + LC – GAnt
Example Application
The maximum EIRP for a LoRa device operating at 868 MHz is +16 dBm. The maximum antenna gain is specified to be 2.15 dBi.
Using these numbers and a cable loss of 2 dB, the maximum output power from the transmitter is restricted to 0.038 Watt.