Sailboat ratio calculator
Enter the figures from a boat's spec sheet: sail area, displacement, ballast, waterline length, overall length and beam. The calculator returns the classic design ratios used to compare how powered, heavy, stiff and comfortable different boats are.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Choose metric or US units.
- Enter sail area, displacement and ballast from the spec sheet.
- Enter the waterline length, overall length and beam.
How it works
All ratios use US units internally (ft², lb, ft); metric inputs are converted first. 64 lb per cubic foot is the weight of seawater.
- SA/D = sail area ÷ (displacement ÷ 64)^(2/3);
- D/L = (displacement ÷ 2,240) ÷ (0.01 × LWL)³;
- ballast ratio = ballast ÷ displacement;
- CSF = beam ÷ (displacement ÷ 64)^(1/3);
- comfort ratio = displacement ÷ (0.65 × (0.7 × LWL + 0.3 × LOA) × beam^1.333).
Example
A 34-footer: 540 ft² of sail, 11,000 lb displacement, 4,400 lb ballast, LWL 28 ft, beam 11 ft:
- 11,000 ÷ 64 = 171.9 ft³; to the power 2/3 = 30.9; SA/D = 540 ÷ 30.9 = 17.5;
- 11,000 ÷ 2,240 = 4.91 long tons; 0.28³ = 0.02195; D/L = 224;
- ballast ratio 4,400 ÷ 11,000 = 40 %;
- CSF = 11 ÷ 5.56 = 1.98; comfort ratio 23.2.
Tips and common mistakes
- Published displacement is often light ship. A loaded cruising boat is heavier, which lowers SA/D and raises D/L.
- Sail area for SA/D is usually the main plus the 100 % foretriangle, not including overlapping genoas or spinnakers.
- Compare boats with figures from the same source and method.
Frequently asked questions
What is a good sail area to displacement ratio?
SA/D compares sail power with weight. Values around 16–18 are often called moderate cruising boats, under about 15 underpowered or heavy, and over 20 performance or racing boats. Light-air sailing favours higher values.
What does the displacement/length ratio tell me?
D/L = (displacement in long tons) ÷ (0.01 × LWL in feet)³. It shows how heavy a boat is for its waterline: under about 100 is ultralight, 200–300 a moderate cruiser, and over 350 heavy.
What is the capsize screening formula?
CSF = beam ÷ (displacement in cubic feet)^(1/3). Values under 2 are usually taken as more suitable for offshore sailing. It ignores ballast placement and hull shape, so it is only a first screen.
What is the comfort ratio?
Ted Brewer's ratio estimates how quick and violent a boat's motion is in a seaway: displacement ÷ (0.65 × (0.7 × LWL + 0.3 × LOA) × beam^1.333). Higher means a gentler motion; light coastal boats are often in the 20s, heavy bluewater boats in the 40s or more.