Hull Speed Calculator (Feet & Metres)
Enter waterline length in feet or metres. Calculate the classic hull-speed reference in knots, mph and km/h, and understand what it can tell you.
Enter waterline length in feet or metres. Calculate the classic hull-speed reference in knots, mph and km/h, and understand what it can tell you.
Start with your boat’s waterline length.
1.34 × √waterline feet → knots
For metres, this calculator converts length to feet first: feet = metres ÷ 0.3048. It then converts knots to mph and km/h. A 25 ft waterline, or 7.62 m, gives 6.7 knots, about 7.7 mph or 12.4 km/h.
The classic rule relates waterline length to wave-making behavior. It does not calculate a boat’s maximum speed. Hull shape matters, and some slender displacement hulls exceed this reference. See naval architect Julian Wolfram’s explanation of hull speed and hull form.
Use the number as context when comparing displacement boats, not as a universal transition to planing or a prediction for a catamaran, kayak or powerboat.
| LWL in feet | LWL in metres | Reference speed |
|---|---|---|
| 15 | 4.57 | 5.2 knots |
| 20 | 6.10 | 6.0 knots |
| 25 | 7.62 | 6.7 knots |
| 30 | 9.14 | 7.3 knots |
| 36 | 10.97 | 8.0 knots |
| 40 | 12.19 | 8.5 knots |
| 45 | 13.72 | 9.0 knots |
| 50 | 15.24 | 9.5 knots |
| 60 | 18.29 | 10.4 knots |
Calculated with the fixed 1.34 coefficient. These are formula examples, not measured boat speeds.
Look for LWL in your boat’s specifications: the length at the waterline. Overall length (LOA) can include overhangs and is a different measurement. Use a documented waterline length for the boat’s loading condition rather than guessing from the model name.
For example, if a hypothetical boat is 30 ft overall but has a 25 ft waterline, enter 25 ft. Using 30 ft would give 7.3 knots instead of 6.7 knots.
This length-only formula does not use engine load, wind, sea conditions or fuel burn. It therefore cannot select your most economical cruising speed or promise a passage time. Compare your boat’s observed speed and fuel use under comparable conditions, then use the fuel cost calculator to plan a trip.
No fixed percentage of hull speed is assumed to be the best cruise setting. Use boat-specific performance information and operating limits.
Yes. The classic number is a reference, not an absolute barrier. Hull form and operating conditions influence performance; waterline length alone cannot set a maximum speed.
Yes. Select metres beside the length field. The calculator converts metres to feet before applying 1.34 × the square root of waterline length.
It calculates the traditional reference used for displacement sailboats. It does not predict speed under a particular sail plan, wind strength or sea state.
Do not use this result as their top speed or replace the coefficient with a larger number to invent a prediction. Use performance data for the actual hull and engine setup.
You can calculate the length-based reference, but narrow hulls can exceed it. It is not a reliable maximum-speed forecast for those craft.
No. The formula uses a square root: doubling length increases the reference by about 41%, all else in the formula unchanged.
Weight is not an input. Loading can affect the actual waterline and performance, but this simple calculation does not model those effects.
Last Updated:
Reviewed by Premium Boatcare Team