Sail Area-Displacement Ratio Calculator
The sail area-displacement ratio is naval architecture's version of a power-to-weight ratio for sailboats. It sets the driving sail area against the boat's displacement, so a higher value indicates a boat that carries more sail for its weight, with livelier acceleration and better light-air performance. The ratio is the sail area in square feet divided by the displacement volume in cubic feet raised to the two-thirds power, which keeps it dimensionless. This calculator converts displacement weight to volume using sea water at 64 pounds per cubic foot, then returns the displacement volume and the sail area-displacement ratio for any consistent set of figures you enter.
Sail area-displacement formula
Displacement volume = displacement (lb) / water density (lb/cu ft)
Volume to the two-thirds = displacement volume ^ (2 / 3)
SA/D ratio = sail area (sq ft) / volume to the two-thirds
Sea water is taken as 64 pounds per cubic foot by default; use about 62.4 for fresh water. The two-thirds power converts the volume to an area-equivalent so the ratio is dimensionless.
Interpreting the ratio
- Higher values mean more sail per unit weight, favouring acceleration and light air.
- Heavy traditional cruisers often sit below 15.
- Typical cruisers fall roughly between 15 and 18.
- Performance cruisers and racers generally exceed 18, sometimes well beyond.
- Compare boats only when sail area is measured the same way for each.
Sail area-displacement: frequently asked questions
What is the sail area-displacement ratio?
The sail area-displacement ratio compares a sailboat's sail area to its displacement, acting like a power-to-weight ratio under sail. It is the sail area in square feet divided by the displacement volume in cubic feet raised to the two-thirds power. A higher ratio means more sail driving a given weight, so more acceleration and lighter-air performance.
How is the ratio calculated?
Convert displacement weight in pounds to a volume in cubic feet by dividing by 64, the weight of a cubic foot of sea water. Raise that volume to the two-thirds power, then divide the sail area in square feet by the result. The two-thirds power makes the ratio dimensionless by matching the area dimensions.
What is a typical value?
As a rough guide, cruising sailboats often fall around 15 to 18, performance cruisers around 18 to 22, and racing boats above that. Heavy traditional cruisers can sit below 15. Treat these as broad bands; design intent, rig type, and how sail area is measured all affect comparisons.
Why divide displacement by 64?
Sea water weighs about 64 pounds per cubic foot, so dividing a displacement in pounds by 64 gives the volume of water the hull displaces in cubic feet. This is exactly the volume the floating boat pushes aside, which is the relevant figure for the ratio. Fresh water is nearer 62.4 pounds per cubic foot.
Which sail area should I use?
Conventionally the ratio uses the working sail area, often the mainsail plus 100 percent of the foretriangle. Different sources measure sail area differently, including or excluding overlap and downwind sails, so use a consistent definition when comparing boats. Enter the sail area figure that matches your reference.
Official sources
- NIST Digital Library of Mathematical Functions: powers and roots.
- U.S. Coast Guard Navigation Center: vessel and navigation references.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.