Sail Area to Displacement Ratio Calculator
The sail area to displacement ratio (SA/D) is a classic dimensionless yardstick of a sailboat's power-to-weight: how much sail it carries relative to its size. It divides the sail area by the displacement volume raised to the two-thirds power, which keeps the ratio non-dimensional. Enter the upwind sail area in square feet and the displacement in pounds; the calculator converts weight to volume using an editable sea water weight and returns the SA/D.
SA/D ratio formula
Displacement volume (ft^3) = displacement (lb) / water weight (lb/ft^3)
SA/D = sail area (ft^2) / (displacement volume)^(2/3)
The two-thirds power converts a volume to an area-like quantity, so the ratio is dimensionless. Sea water weighs about 64 pounds per cubic foot; fresh water about 62.4.
Interpreting SA/D
- Under about 16: heavy displacement cruiser, more sail needed in light air.
- About 16 to 20: typical cruiser or cruiser-racer.
- Above about 20: performance or racing design.
- Always compare boats using the same sail-area convention.
SA/D ratio: frequently asked questions
What is the sail area to displacement ratio?
The sail area to displacement ratio (SA/D) is a dimensionless number comparing a boat's sail power to its weight. It is the sail area in square feet divided by the displacement volume in cubic feet raised to the two-thirds power: SA/D = sail area / (displacement volume)^(2/3). Higher values mean more sail power per unit of weight.
How is displacement volume found from weight?
Displacement volume in cubic feet equals the boat's weight in pounds divided by the weight of one cubic foot of sea water, about 64 pounds. This calculator does that step for you, using an editable water weight, then raises the volume to the two-thirds power.
What is a typical SA/D value?
As a general guide, values under about 16 indicate a heavy cruiser, around 16 to 20 a typical cruiser or cruiser-racer, and above about 20 a performance or racing design. These are rules of thumb, not fixed thresholds, and depend on how sail area is measured.
Which sail area should I enter?
SA/D is normally computed from the nominal upwind sail area: the mainsail plus the 100 percent foretriangle (jib area to the forestay), not including overlapping genoas or downwind sails. Use a consistent measurement when comparing boats.
Official sources
- NOAA: sea water density (water weight per cubic foot).
- NIST: unit conversions (foot, pound).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.