The Line of Sight calculator will determine both the "line of sight distance" and "radio horizon / service range" of an antenna placed at a specific height above the ground, represented by "antenna height (h)" in this equation.
The calculation assumes there are no obstructions (trees, buildings, etc.) between the antenna and the horizon and does not take into account the potential effects of fading or other interference.
The radio horizon / service range of an antenna will always be slightly further than the straight line-of-sight distance to the horizon. This is due to the refractive effect of the Earth's atmosphere, which causes RF signals to propagate in a slightly curved line over long distances, pushing out the effective horizon distance.