Boat Speed Calculator
Estimate the theoretical top speed of a displacement boat. Enter the waterline length (feet or meters) and pick a speed/length coefficient to get hull speed in knots, mph, and km/h.
Example: with Waterline length (LWL) 26 · Length unit feet · Speed/length coefficient 1.34 — classic hull speed → Hull speed: 6.83 knots.
- In mph7.86 mph
- In km/h12.65 km/h
Computed by the calculator below using its default values. Change any input to see your own numbers.
Hull speed (knots) = 1.34 × square root of the waterline length in feet. It marks where a displacement hull must climb its own bow wave, which takes sharply more power to exceed.
What hull speed means
A displacement boat pushes water aside as it moves, creating a bow wave and a stern wave. As speed rises, those waves stretch out until their crests sit at the bow and stern — a wavelength equal to the waterline length. At that point the boat is effectively trapped in the trough of its own wave, and going faster demands a steep jump in power. That threshold is hull speed.
The 1.34 coefficient comes from wave physics (the speed/length ratio) and fits most conventional hulls. Heavy, full-bodied cruisers behave more conservatively near 1.1, while light, narrow, or planing-capable hulls can exceed 1.34 and, with enough power, break free onto a plane entirely.
How it’s calculated
Hull speed in knots = coefficient × square root of waterline length in feet, with the classic coefficient 1.34. Meters convert at 1 m = 3.28084 ft. Knots convert to mph at 1.15078 and to km/h at 1.852.
Applies to displacement hulls, not planing boats, which can exceed it with enough power. Real speed also depends on hull shape, weight, and conditions.
Hull speed by waterline length (coefficient 1.34)
| LWL (ft) | Hull speed (knots) | Approx mph |
|---|---|---|
| 16 | 5.36 | 6.17 |
| 20 | 5.99 | 6.90 |
| 25 | 6.70 | 7.71 |
| 30 | 7.34 | 8.44 |
| 36 | 8.04 | 9.25 |
| 40 | 8.47 | 9.75 |
Computed as 1.34 × square root of LWL in feet; knots to mph at 1.15078. Rounded.
Common mistakes
- Applying hull speed to a planing powerboat, which is designed to climb past it onto a plane.
- Using overall length instead of waterline length — only the wetted waterline sets the wave.
- Expecting a little more horsepower to add a lot of speed near hull speed; the power curve turns nearly vertical there.
Frequently asked questions
What is the hull speed formula?
Hull speed in knots equals 1.34 times the square root of the waterline length in feet. For a 25-foot waterline, that is 1.34 × 5 = 6.7 knots.
Why is it based on waterline length, not overall length?
The bow and stern waves are set by the length of hull actually in the water. Overhangs above the waterline do not lengthen the wave, so they do not raise hull speed.
Can a boat go faster than hull speed?
Displacement hulls can only nudge past it with a large power increase. Light or planing hulls can break free of the bow wave and plane, running well above the formula speed.
How do I convert knots to mph?
Multiply knots by 1.15078. So 6.7 knots is about 7.7 mph, and by 1.852 for about 12.4 km/h.