How this calculator works
Kite power depends on sail area and on the square of wind speed, while a rider needs a roughly fixed amount of power to plane. That gives a simple rule of thumb: kite area in square metres is about 2.2 times body mass in kilograms divided by wind speed in knots. A 75 kg rider in 18 knots therefore needs about 9.2 m squared, and the same rider in 12 knots needs about 13.8 m squared. The page adjusts the result slightly for experience: beginners use a little more area in light wind for stability, while advanced riders can work a smaller, faster kite. The working range is plus or minus ten percent of the suggested size.
Formula
Kite size (m2) = 2.2 x body mass in kg / wind speed in knots, multiplied by 1.1 for a beginner, 1.0 for an intermediate or 0.95 for an advanced rider. Working range = suggested size x 0.9 to suggested size x 1.1. Wind in knots = wind in m/s / 0.514444.
Worked example
A 75 kg intermediate rider in 33 km/h wind has 33 / 1.852 = 17.82 knots. The suggested size is 2.2 x 75 / 17.82 = 9.26 m squared. At 12 knots the same rider needs 2.2 x 75 / 12 = 13.75 m squared. At 24 knots the size falls to 6.88 m squared.
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Frequently asked questions
Is this formula exact?
No. It is a widely used rule of thumb for a twin-tip board and an intermediate rider. Kite design, board type, rider weight distribution, current and gusts all shift the right size by up to a metre or so.
Why does the size fall so quickly as wind rises?
Aerodynamic force rises with the square of wind speed, so a small increase in wind delivers much more power. Halving the appropriate area as wind doubles is roughly what keeps the load on the rider constant.
What should I do in gusty wind?
Size for the gusts, not the average, or choose the smaller of two kites. A gust can add over fifty percent power on a windy day, and depowering a slightly too large kite is easier than finding power in one that is too small.
Do trainer kites use the same rule?
No. Trainer kites are much smaller, usually 1.5 to 3 m squared, and are sized for learning control rather than for riding. Use the manufacturer recommendation for a trainer and treat this calculator as a guide for a full-size kite.