Disclaimer: This is only an estimation! Take the result with a grain of salt.
See this post about what dB is and how it affects FPV range.
This calculator assumes perfect conditions. There are many factors that can affect FPV range, so you are likely going to get less range in real life.
FPV Range Calculator
|VTX Antenna Gain||dBi|
|VRX Antenna Gain||dBi|
VTX Power mW to dB Look Up Table