What this calculator does
Distance to the Horizon works out geometric sight distance to the horizon from an observer height above a sphere. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
Formulad = √((r+h)² − r²) (pure geometry, no atmospheric refraction)
The inputs explained
| Field | What to enter |
|---|---|
| Observer height above surface (m) | A number, measured in m. Starts at 1.7. |
| Planet radius (Earth ≈ 6371000) (m) | A number, measured in m. Starts at 6371000. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with observer height above surface
Every other input is held at the calculator’s starting values while observer height above surface varies. Select any row to load that scenario into the calculator.
How the answer changes with observer height above surface