Far-field, unobstructed free space with gains in the link direction and matched polarization. Antenna mismatch and additional attenuation must be included in system loss. This does not predict indoor, terrain or multipath coverage. ITU-R P.525.
Free Space Path Loss (FSPL):
$FSPL_{dB} = 20 \log_{10}(R_{km}) + 20 \log_{10}(f_{MHz}) + 32.44$
Friis Equation (Logarithmic / dB form):
$P_r (dBm) = P_t (dBm) + G_t (dBi) + G_r (dBi) - FSPL_{dB} - L_{misc}$
This ideal equation assumes free space propagation with no multi-path, reflections, or atmospheric absorption.