Hull speed can be calculated by the following formula:
where
: is the length of the waterline in feet, and
: is the hull speed of the vessel in knots
If the length of waterline is given in metres and desired hull speed in knots, the coefficient is 2.43 kn·m. The constant may be given as 1.34 to 1.51 knot·ft in imperial units (depending on the source), or 4.50 to 5.07 km·h·m in metric units, or 1.25 to 1.41 m·s·m in SI units.
The ratio of speed to  is often called the "speed/length ratio", even though it is a ratio of speed to the square root of length.