Light Visibility Range Calculator
Navigational lights are characterized by their geographic range (determined by height and Earth curvature) and their luminous range (determined by intensity and atmospheric visibility). This calculator determines both ranges. Geographic range uses the standard formula 1.17 * sqrt(height in feet) for each horizon distance (observer and light), summed. Luminous range is the nominal range from the Light List adjusted for prevailing atmospheric visibility using a simplified model. The lesser value is the predicted visible range. Enter observer eye height, light focal plane height (from the Light List), and nominal range.
Light range formulas
Geographic range = 1.17 * (sqrt(eye height) + sqrt(light height)) nm
Luminous range adjustment (simplified):
Luminous range = Nominal range * (Visibility / 10)^0.5
(Approximate: valid near 10 nm nominal visibility)
Visible range = min(Geographic range, Luminous range)
For precise luminous range from nominal range at different visibilities, use Bowditch Table 12 or the USCG Light List visibility curves.
Using the Light List
- The USCG Light List is published annually; always use the current edition for accurate light data.
- Charts may show outdated light characteristics; the Light List takes precedence for precise planning.
- Light characteristics (Fl, Oc, Iso, etc.) help identify the light; range determines when you will see it.
- At night, note the time you first see a light and compare to the predicted range; differences indicate atmospheric conditions.
Light visibility range: frequently asked questions
What is the difference between geographic range and luminous range?
Geographic range is the maximum distance at which a light can be seen based purely on the geometry of the Earth's curvature and the heights of the observer and the light. Luminous range is the maximum distance at which the light's intensity is sufficient to be visible in prevailing atmospheric visibility. The actual visibility range is the lesser of the two.
What is nominal range?
Nominal range is the luminous range of a light in a meteorological visibility of 10 nautical miles (10 nm), as published in the US Coast Guard Light List. It is used as a standard reference for comparing light intensity. In better visibility, the luminous range exceeds nominal range; in fog, it is much less.
Where do I find light heights and nominal ranges?
The USCG Light List (published annually and available online at navcen.uscg.gov) lists the focal plane height (feet above water) and nominal range (nm) for all US aids to navigation. Charts also show the light's characteristic and height.
How does atmospheric visibility affect the visible range?
In 10 nm visibility, luminous range equals nominal range. In 5 nm visibility, the luminous range is approximately 0.5 times nominal range. In 1 nm visibility (thick fog), the luminous range may be only 10-20% of nominal range. Use the NOAA Visibility in Fog table or Bowditch Table 12 for precise conversions.
What is dipping range?
Dipping range is the distance at which a light first appears on the horizon from the observer's position. It equals the geographic range (sum of observer horizon distance and light horizon distance). The light will appear to dip below the horizon as the vessel moves away, hence the term.
Official sources
- US Coast Guard Light List: USCG Light Lists (Volumes I-VII).
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.