calculating maximum transmission distance of radio module based on dbm and open air
In free space and assuming an isotropic antenna at each end: -


Path loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)


Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.

If you know the gains of the antennas you can reduce the path loss by a few dB.


Power (dBm) needed by a receiver is -154&nbsp;dBm + 10\$log_{10}\$(data rate) 


This is based on an empirical derivation and assumes an ambient temperature of 300 kelvins and a reasonably low data error rate. This is a baseline value.

But on earth, no matter what the terrain appears to be, there will be added attenuations that are really difficult to account for and describe here. There's a thing called fade margin and this, as a rule of thumb basically says - try to ensure your received power is at least 20 dB greater than its baseline sensitivity - this means that if you designed a receiver requiring -120 dBm you should expect to receive -100 dBm on a good day.

Information taken from my answer here.