How this calculator works
A windsurf sail converts wind into forward drive, and the force rises with sail area and with the square of wind speed. The page uses a common freeride rule of thumb: base sail area is 0.12 times body mass in kilograms divided by the square root of wind speed in knots divided by ten. Doubling the wind raises the square root term by about 41 percent, so the area needed falls by about 29 percent. The result is then adjusted for experience, with beginners using slightly less sail and advanced riders slightly more. Board volume starts at the sailor mass in kilograms plus a comfort allowance: about 50 litres for a beginner, 25 litres for an intermediate and none for an advanced rider, which matches the usual advice that a beginner board should be substantially larger than body weight.
Formula
Sail area (m2) = 0.12 x body mass in kg / square root(wind in knots / 10), multiplied by 0.9 for a beginner, 1.0 for an intermediate or 1.05 for an advanced rider. Sail range = suggested area x 0.92 to x 1.08. Board volume (L) = body mass + 50 for a beginner, + 25 for an intermediate or + 0 for an advanced rider. Wind in knots = wind in m/s / 0.514444.
Worked example
An 80 kg intermediate sailor in 25 km/h wind has 25 / 1.852 = 13.5 knots. The sail area is 0.12 x 80 / square root(1.35) = 9.6 / 1.162 = 8.26 m squared, and the range is about 7.6 to 8.9 m squared. The starting board volume is 80 + 25 = 105 litres. At 20 knots the same sailor needs 0.12 x 80 / square root(2) = 6.79 m squared.
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Frequently asked questions
Why does a heavier sailor need a bigger sail?
A heavier sailor displaces more water and needs more drive to reach planing speed. The area grows in direct proportion to mass in this rule, so a ten kilogram difference is worth about one square metre in the same wind.
How does board volume relate to body mass?
A board must float the sailor and the rig before planing. A beginner needs enough excess volume to stand and uphaul comfortably, often 40 to 60 litres above body mass. As skill rises, the board can shrink toward body mass or below for sinker boards.
What about fin size?
Fin size should match sail size and board width. A fin that is too small spins out under a large sail, and one that is too large adds drag. The calculator does not model fins, so check the board manufacturer recommendation alongside the sail and volume figures.
Is this useful for wing foiling?
The sail logic is similar but the wing sizes are smaller and the board volume is much larger for a given rider. Treat this page as a windsurfing guide and use wing-specific tables for wing foiling.