What the Numbers Mean
Each antenna has its own horizon. Two stations can usually hear each other directly when their two horizons add up to at least the distance between them. That is why a repeater on a hilltop covers so much ground: its height does most of the work for everyone.
The calculator assumes smooth, open ground and the normal slight bending of radio waves in the atmosphere, which carries them a little past the line of sight. Hills, buildings and trees shorten the real range. Standing on a mountain, or the band opening through tropospheric ducting or sporadic E, can stretch it far beyond these figures.
Heights are above the ground around you, not above sea level. A 30-foot mast on a hilltop that stands 200 feet above the surrounding land counts as 230 feet.