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Sailing Hull Speed Calculator

Enter the waterline length in feet to get the hull speed.

How it is calculated

The tool uses the classic rule of thumb for displacement hulls: hull speed (kn) = 1.34 × √LWL, where LWL is the length of the boat at the waterline in feet. One knot is one nautical mile per hour, and the tool also converts the answer to kilometres per hour with km/h = knots × 1.852. Both figures are shown to two decimals. The length must be above zero. Only one input matters, so the hull speed grows with the square root of length: a boat four times as long is only twice as fast. This is a theoretical ceiling for a displacement monohull, not a prediction of average cruising speed. It ignores weight, hull shape and sail area, and light or planing hulls can exceed it.

Worked example

A sailboat with a 30 ft waterline. √30 = 5.477. Hull speed = 1.34 × 5.477 = 7.34 kn. In kilometres per hour: 7.34 × 1.852 = 13.59 km/h. For a 25 ft waterline the same formula gives 6.70 kn, so adding 5 ft of waterline buys only about 0.6 knots. Below hull speed the boat usually moves easily; trying to exceed it means pushing against a rapidly growing wave resistance.

Try this example →

Common mistakes

  • Entering overall length (LOA) instead of waterline length. LWL is usually noticeably shorter than LOA, so LOA overstates the speed.
  • Entering the waterline in metres. The formula needs feet; multiply metres by 3.281 first.
  • Treating the figure as the speed the boat will actually sail. It is an upper guide for displacement hulls, and real passage speeds depend on wind, sail trim and load.
  • Applying it to planing boats, catamarans or very light boats. They are not limited this way and may go much faster.
  • Confusing knots with miles per hour. A knot is a nautical mile per hour, about 1.15 mph.

About this calculator

Hull speed is the theoretical speed at which a traditional displacement-hull boat's own bow and stern wave crests match its waterline length, creating a wave "trough" the hull sits in and making it disproportionately hard to go faster without extra power or lifting out of the water; the widely used rule of thumb is 1.34 × √(LWL in feet) knots.

Naval architects and sailors use it as a quick ballpark for how fast a given monohull sailboat can realistically cruise under displacement conditions — a boat with a 25-foot waterline tops out around 6.7 knots — though modern planing hulls, catamarans and boats that surf down waves can and do exceed it.

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