Metric first, imperial when you need it
SailboatReference shows every specification in metric units: metres, square metres, kilograms and litres. Many designers, brokers and older brochures use feet, square feet, pounds and gallons, and the sailing ratios were originally defined in imperial units. This converter turns the numbers you find in a listing or a brochure into the units you are used to, in both directions. Type in either field; the other updates as you type.
The conversion factors
- Length: 1 m = 3.2808 ft, and 1 ft = 0.3048 m exactly. Used for length overall, waterline, beam, draft and mast height.
- Area: 1 m² = 10.764 ft², and 1 ft² = 0.0929 m². Used for sail area, including the I, J, P and E rig measurements once multiplied out.
- Weight: 1 kg = 2.2046 lb, and 1 lb = 0.4536 kg. Used for displacement and ballast. A metric tonne is 1,000 kg or 2,205 lb; a long ton, used in the D/L ratio, is 2,240 lb or 1,016 kg.
- Volume: 1 L = 0.2642 US gallons, and 1 US gallon = 3.785 L. Used for fuel and water tanks. An imperial (UK) gallon is larger, 4.546 L.
Common pitfalls
Gallons are the most common source of errors: American brochures use US gallons, older British ones imperial gallons, and a tank of "50 gallons" can hold 189 or 227 litres depending on which is meant. Displacement can be given in short tons (2,000 lb), long tons (2,240 lb) or metric tonnes, and brochures do not always say which. Sail area is sometimes quoted with an overlapping genoa or even a spinnaker, which makes it look larger than the mainsail-plus-foretriangle figure the ratios expect.
Rounding matters less than people think. A beam of 3.9 m is 12.8 ft, and the difference between 12.8 and 12.79 ft disappears in the uncertainty of the published figure itself. Builders measure displacement differently: light ship, half load or fully loaded. That difference is usually much larger than any rounding.
Using the numbers in the calculators
You do not need to convert anything before using our calculators: each one has a metric and imperial switch and converts the values you have already entered. The ratios themselves have the same value in both systems, except the displacement/length ratio and the Bruce number, whose formulas mix units; the calculators handle that for you. Try the hull speed, SA/D or comfort ratio calculators.
Sources
The foot and the pound are defined exactly in metric terms by the 1959 international yard and pound agreement; the US gallon is defined as 231 cubic inches. See our sources and methodology for how specifications are collected.