What SA/D measures
The sail area to displacement ratio (SA/D) is the sailing equivalent of a power-to-weight ratio. Sail area is the engine and displacement is the mass it has to move, so a higher SA/D means more drive for every tonne of boat. It is the best single number for comparing performance in light and moderate wind, which is when most sailing happens.
The formula
SA/D divides the sail area by the displacement volume raised to the power two-thirds, which turns a volume into an area, so the ratio has no units and the result is the same in metric and imperial.
- Metric:
SA/D = SA (m²) ÷ (Disp (kg) ÷ 1025)2/3, where 1025 kg/m³ is the density of seawater. - Imperial:
SA/D = SA (ft²) ÷ (Disp (lb) ÷ 64)2/3, where 64 lb/ft³ is the same density.
The sail area is the mainsail plus 100% of the foretriangle (I × J ÷ 2), without overlapping genoas or downwind sails. Using the reported area of a large genoa inflates the number, so compare like with like.
How to read it
- Under 16: under-canvassed. Typical of heavy cruisers and motorsailers; they need a good breeze to come alive.
- 16 to 20: moderate. Most production cruising boats sit here, a sensible balance for cruising with a small crew.
- 20 to 24: lively. Performance cruisers and cruiser-racers that sail well in light air.
- Over 24: high-powered. Racing boats and light performance designs, which have to reef early.
The calculator also shows where your value sits among boats of the same hull type and length in our catalogue, because what is lively for a 9 m boat can be ordinary for a 15 m one.
Limitations
SA/D ignores stability. A boat with lots of sail but little righting moment has to reef early and cannot use its power in a breeze; read SA/D together with the ballast ratio and the displacement/length ratio. Displacement figures are usually the builder's light-ship weight; a cruising boat with fuel, water, stores and crew can weigh 15 to 25% more, which lowers the real SA/D noticeably. For multihulls, the monohull bands are optimistic: a catamaran needs a higher SA/D to perform, and the Bruce number is the usual yardstick.
Sources
The ratio and its bands are those in common use by designers such as Ted Brewer and by sailboatdata.com, whose published SA/D values this calculator reproduces to within half a percent. See our sources and methodology for how the catalogue figures are collected.