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A simple jib or mainsail without roach approximates a triangle: half the base times height (luff).
Area = (foot × luff) / 2Modern mainsails have roach extending beyond the straight leech. Add roach area using the roach percentage or measure the leech curve.
Total mainsail area ≈ (P × E / 2) × (1 + roach% / 100)Genoa area scales with LP (luff perpendicular) as a percentage of J dimension. A 150% genoa has LP = 1.5 × J.
Genoa area ≈ (LP × I) / 2 (LP in % of J × J)Updated: July 2026
P (luff) 38 ft, E (foot) 13 ft: area = 38 × 13 / 2 = 247 sq ft. With 8% roach: ~267 sq ft total mainsail area.
I (foretriangle height) 42 ft, J (base) 12 ft: 100% jib area = 12 × 42 / 2 = 252 sq ft. Total working sail area ~519 sq ft.
LP = 18 ft (150% of J=12), I = 42 ft: genoa area = 18 × 42 / 2 = 378 sq ft. Significantly more drive in light winds than 100% jib.
E is the mainsail foot dimension, not always full boom length. Loose-foot mainsails and short-footed racing mains have E less than boom length.
Roach adds 5–15% area beyond the triangle approximation. Rating rules and sail balance calculations need roach included for accuracy.
Sail area drives boat speed and balance. This calculator computes sail area in square feet or square meters from luff, leech, foot, and roach measurements for mainsails, jibs, genoas, and spinnakers.