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Sail Area / Displacement Ratio Calculator

Calculate a sailboat's sail area to displacement ratio (SA/D = SA ÷ (disp/64)^⅔), interpret it from under-canvassed cruiser to racer, see the sail area needed for a target ratio, the upwind versus downwind sail plan, and how loading the boat for cruising lowers the number.

Upwind sail area (main + 100% foretriangle)

Displacement

Cruising payload added (crew, stores, water, gear)

Target ratio for the sail-plan check

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How it works

SA/D = sail area (ft²) ÷ (displacement (lb) ÷ 64)^⅔ — i.e. sail area over the ⅔ power of displaced volume in ft³ of sea water

Dividing sail area by displacement raised to the two-thirds power makes the ratio dimensionless — sail area is an area, displacement a volume — so boats of any size can be compared on how much driving power they carry per unit of weight. Around 16 marks the boundary between conservative and moderate cruisers, 20 the start of performance-cruiser and racer territory, and dinghies and multihulls run past 30. Because the ratio uses light-ship displacement in the brochure, a real cruising boat with tanks full and lockers loaded is typically 10–15% lower than advertised, which is why heavily loaded voyagers feel sluggish and why owners add light-air sails rather than heavier gear.

Worked example

A 36-footer with 750 ft² of upwind sail and 18,000 lb displacement has SA/D = 750 ÷ (281.25)^⅔ = 17.5 — a moderate cruiser; loaded with 2,500 lb of crew and stores it drops to 16.0. To reach 18 as designed it would need about 773 ft².

Frequently asked questions

Which sail area do I use?

Convention is mainsail plus 100% foretriangle (I × J ÷ 2), not the actual genoa. Using a 150% genoa inflates the ratio by 15–20% and makes brochure comparisons meaningless.

Why 64?

64 lb is the weight of a cubic foot of sea water, so displacement ÷ 64 is the volume of water the hull displaces in cubic feet; its ⅔ power is an "area" comparable with the sail area.

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