Use this sailing calculation for planning or education when the vessel, units, reference directions, environmental conditions, and assumptions are explicitly known.

Visibility (NM)

Required number in NM. Horizontal visibility distance.

The cited overview of Maritime safety supplies background for the terminology and domain context used by this tool.^[1]

Classify horizontal visibility in nautical miles into fog, poor, moderate, or good bands used in marine forecasts. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

Category

Returned string. One of fog, poor, moderate, or good.

• Vessel design and loading, sea state, wind, current, tide datum, weather, chart accuracy, equipment, crew, and safety margins can make real conditions differ from a simplified result.
• Visibility must be at least 0.
• Use finite inputs in the displayed units, preserve source measurements and assumptions, and independently verify consequential decisions.

Cross-check current charts, tide and weather services, instrument data, vessel manuals, and applicable navigation rules, and retain a conservative alternative plan.

• Horizon Range Calculator

  Compute the distance to the visible horizon in nautical miles as one point one seven times the square root of the observer height in feet.

• Light Range Corrected

  Correct a navigation light's nominal range for prevailing meteorological visibility using Allard's inverse-square and atmospheric-transmission relation. Nominal range is defined at ten nautical miles of visibility.

• Ceiling Category Lookup

  Classify a cloud ceiling as low below five hundred feet, marginal below three thousand feet, or otherwise fine for visual flying rules.