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

Full sail area (m2)

Required number in m2. Total unbattened sail area before reefing.

Reefed sail area (m2)

Required number in m2. Remaining sail area after reefing.

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

Compute the percentage of sail area reduced when sailing on a reefed headsail or mainsail. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

Reef percentage (%)

Returned number in %. Fraction of sail area reduced by reefing.

• 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.
• Full sail area must be at least 0.
• Reefed sail area 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.

• Sail Area Calculator

  Compute the triangular mainsail area from the luff (P) and foot (E) measurements.

• Mainsail Area Calculator

  Compute the triangular mainsail area using mainsailAreaM2 = 0.5 * luffLengthMeters * footLengthMeters.

• Jib Overlap Percentage

  Compute jib area as a percentage of foretriangle area. This is an area-ratio proxy; formal LP overlap percentage requires the luff-perpendicular and J dimensions.