Sail Area to Displacement (SA/D) Calculator

Sail area to displacement is a dimensionless power-to-weight figure for sailboats. It divides sail area in square feet by the displacement volume in cubic feet raised to the two-thirds power. Raising volume to two thirds converts it to an area-like quantity, so the ratio compares boats of different sizes on a fair basis.

First the displacement in pounds is divided by 64 to get a volume in cubic feet. That volume is raised to the two-thirds power, and the sail area is divided by the result. For 500 square feet on 10,000 pounds, the volume is 156.25 cubic feet and the SA/D works out to about 17.24.

As a rough guide the tool labels under 16 a Heavy cruiser, 16 up to 20 a Moderate cruiser/racer, 20 up to 24 a Performance boat, and 24 or more a Racer. The cutoffs are conventions, not laws, so a boat near a boundary can reasonably be described either way.

The ratio needs a volume, but displacement is quoted as a weight. A cubic foot of seawater weighs about 64 pounds, so dividing pounds by 64 gives the volume of water the hull pushes aside. Freshwater is nearer 62.4 pounds per cubic foot, but the SA/D convention uses the seawater figure of 64.

Use the mainsail area plus the area of the 100 percent foretriangle, the standard designers publish. Do not add the overlap of a genoa or the extra cloth of a spinnaker, because the ratio is defined against that nominal working sail area, not the largest sails you might fly.

Both usually appear in a boat's published specifications, brochure or a sailboat database. Displacement is the design or half-load weight; if you only have a loaded cruising weight it will read a little heavier and lower the ratio slightly.

No. A high ratio means more driving power for light air and quicker acceleration, but it also means the boat heels sooner and needs reefing earlier as the breeze builds. Cruisers often prefer a lower, more forgiving ratio, while racers chase a higher one.

Both fields must be numbers greater than zero. A blank, zero or negative sail area or displacement shows a short prompt instead of a result. Very large or small but valid numbers are fine, since the formula scales smoothly across boat sizes.

Yes. The whole calculation runs in your browser as simple arithmetic, so the sail area and displacement you enter never leave your device and nothing is stored or uploaded.