For coverage prediction or link budget considerations of wireless communications, various path loss models have been developed. All of those models are based on the free-space path loss, which is given as $$ \left.PL_{\rm{free}}\left(\lambda,d\right)\right|_{\rm{dB}}=10\log_{10}\left(\frac{\lambda}{4\pi d}\right)^{2} $$ with transmitter/receiver-distance $d$, wavelength $\lambda=c/f_{c}$, speed of light $c=3\cdot10^{8}\frac{m}{s}$, and carrier frequency $f_{c}$.

A simple link budget consideration, which is referred to as Friis' law is is given as $$ P_{\rm RX}=P_{\rm TX} \cdot G_{\rm TX} \cdot G_{\rm RX} \cdot PL_{\rm{free}} $$ with transmit power $P_{\rm TX}$, and $G_{\rm TX}, G_{\rm RX}$ being the transmit and receive antenna gains, respectively.

Expressing Friis' law in logarithmic quantities, the received power $P_{\rm RX}$ computes to $$ \left.P_{\rm RX}\right|_{\rm{dBm}}=\left.P_{\rm TX}\right|_{\rm{dBm}}+\left.G_{\rm TX}\right|_{\rm{dB}}+\left.G_{\rm RX}\right|_{\rm{dB}}+\left.PL_{\rm{free}}\right|_{\rm{dB}} $$

In the following, you can learn more about the free-space path loss model, the breakpoint-model, the Okumura-Hata model, and the Motley-Keenan indoor path loss model, with $PL_{\rm{free}}$ in Friis' law being replaced by the path loss of the respective model.