About this tool
The Distance to Horizon Calculator is a free, in-browser tool that tells you how far it is to the visible horizon from any eye height, and how far away a tall object (a lighthouse, mountain, ship or building) first appears over the Earth's curve. Enter your height above the water or ground and the answer updates instantly — nothing is uploaded and no map or GPS is needed.
For the horizon itself it uses the standard geometric approximation d(km) = 3.57 × √h(m), where h is your eye height in metres; in miles this is the equivalent d(mi) = 1.22 × √h(ft). Switching on atmospheric refraction bends light slightly around the curve and raises the constant to about 3.86, extending the visible range by roughly 8%. When you also enter a target height, the tool adds the two separate horizon distances — one for your eye and one for the object's top — so the total is how far apart you can be and still see the object peek over the horizon.
This is the geometric line-of-sight limit only; haze, waves, terrain and light conditions all shorten what you can actually make out. Use it for coastal navigation, spotting summits, planning photography or simply settling the question of how far you can see from a hilltop or a plane window.