What this lighthouse calculator does
This lighthouse calculator estimates how far a lighthouse can be seen over Earth’s curvature. It combines the distance to the horizon from the light itself and the distance to the horizon from the observer’s eye level. If you add a nominal light range, the tool also tells you whether geometry (curvature) or brightness is the limiting factor.
In practical terms, mariners use two ideas: geographic range and luminous range. Geographic range is pure geometry. Luminous range depends on brightness, atmosphere, and visibility conditions. Safe navigation requires respecting both.
How the math works
The calculator uses the horizon distance formula:
d = √((R + h)2 − R2)
- d = distance to horizon
- R = Earth radius
- h = height above sea level
It computes this once for the lighthouse focal plane and once for the observer. The two are then added to get maximum line-of-sight distance. A weather reduction factor is applied to create a more conservative operational estimate.
Input guide
1) Lighthouse focal height
Use the height of the light source above mean sea level, not the tower height from ground to top unless they are the same.
2) Observer eye height
For small boats this is often 1.5–3 m. For larger vessels or bridge wings, it can be much higher.
3) Nominal light range
Optional, but useful. If entered, the calculator compares it against geometric range and returns the more realistic limiting value.
4) Weather reduction factor
A simple way to account for haze, spray, humidity, and non-ideal conditions. In poor visibility, use a higher reduction percentage.
Example scenario
Suppose a lighthouse focal plane is at 45 m and your eye level is 2 m. The geometric range may be around 19 nautical miles. If weather reduction is set to 10%, practical range becomes lower. If the nominal range is only 15 nautical miles, then brightness and atmospheric transmission—not Earth curvature—set the final visibility limit.
Best use cases
- Voyage planning and coastal approach checks
- Training for navigation students
- Pre-briefing for small craft trips
- Photography planning for distant coastal lights
Limitations and safety notes
- Does not replace official nautical charts, light lists, or Notices to Mariners.
- Assumes clear line of sight over open water; terrain and structures can block visibility.
- Atmospheric refraction is simplified through a basic reduction factor.
- Use conservative assumptions in real-world route decisions.
For live navigation, always prioritize certified marine publications, radar/AIS, and good seamanship practices.