Calculate and understand the key performance indicator of your sailboat
"My Boat Plans" is a collection of over 500 boat plans, from simple dinghies to larger cruisers. It includes step-by-step guides, video tutorials, and more.
Download NowThe sail area to displacement ratio is the single most useful number for predicting how a yacht will feel under canvas, because it compares the power the sails can generate against the weight the hull must move. Two boats of the same length can sail utterly differently, and this ratio usually explains why.
It is sail area divided by a displacement volume raised to the two-thirds power, using the cubic-foot equivalent of weight so that a bigger boat's naturally greater area is normalised. The result classifies a rig as light, medium or heavy weather, and tells you whether the boat will be underpowered, balanced or over-canvassed in a given breeze.
Enter the total sail area and the displacement, and the calculator returns the ratio and its interpretation, so you can compare your boat against the classic bands for daysailing, coastal cruising and offshore work.
SA/D = Sail Area ÷ (Displacement ÷ 64)^0.67| SA/D | Character | Behaviour | Typical Boat |
|---|---|---|---|
| Under 18 | Light | Needs breeze, engine often | Full-keel heavy cruisers |
| 18 - 29 | Medium | Balanced all-round | General coastal cruisers |
| 29 - 40 | Heavy-air | Powerful, early reefing | Performance and racing |
| Over 40 | Very light | Fast, over-canvassed soon | Daysailers, skiffs |
Classic SA/D bands and their character.
Formula verified against standard naval-architecture references · Updated October 2026
It depends on the boat's job. Full-keel heavy cruisers often sit below 18 and are genuinely underpowered in light air, general coastal cruisers land around 18 to 29 for a balanced feel, and performance or racing boats push well above 30 for acceleration at the cost of early reefing. Enter your sail area and displacement and the calculator returns the number and tells you which band your rig sits in.
Divide the total sail area in square feet by the displacement in pounds divided by 64, that quotient raised to the two-thirds power. The 64 converts weight to a cubic-foot volume and the exponent scales it to a length so area compares fairly across boat sizes. The calculator runs exactly that and returns the dimensionless ratio.
Often, but only if the hull can carry the power. A higher ratio means more sail for the weight, so the boat accelerates and gets moving in lighter air, but it also heels earlier and needs more skill and reefing to keep flat. Speed also depends on hull form, wetted surface, ballast and stability, so SA/D is a strong predictor of liveliness, not a guarantee of outright pace.
The traditional convention is the mainsail plus a one-hundred-percent headsail, measured to the foretriangle. Including a large genoa, an asymmetric or a spinnaker would inflate the figure and make comparisons meaningless, so use standard working-sail measurements. Consistency is what lets the classic bands describe your boat accurately.
Not dangerous, but potentially frustrating. A low ratio means the boat is over-weighted for its sail plan, so it will be stiff and comfortable in a blow but sluggish and often motor-sailing in light air. Offshore sailors sometimes accept this deliberately for control, but in light-wind cruising it can mean the engine earns its keep far more than planned.
It only measures power versus weight, not the ability to carry that power, so it must be read alongside stability numbers like the ballast-displacement ratio and the Stability Number. A high SA/D on a boat with modest righting moment is a tender, hard-driving machine, whereas the same number on a stiff hull is a genuinely fast, controllable one.
Partly, by adjusting sail area, though displacement is almost fixed. A larger headsail, taller mast or bigger main raises the ratio and adds power, and deep reefs or smaller sails lower it for control. Just keep the whole rig and its loads consistent, since more sail area also raises the forces analysed by the rigging and heeling-moment calculators.