Horsepower to Knots Calculator
Estimate boat speed in knots from engine horsepower and displacement.
This calculator estimates a planing boat's speed from its engine horsepower and weight, using Crouch's classic formula. Results are shown in knots and MPH.
Crouch's Planing Formula
The hull constant C reflects how efficiently the boat planes: around 150 for heavy cruisers, 190 for runabouts, and 220+ for light race hulls. Choose the value that matches your boat type for the best estimate.
How to Use This Calculator
- Enter boat weight (loaded) in pounds.
- Enter engine horsepower.
- Set the hull constant for your boat type.
Worked Example
Frequently Asked Questions
Use Crouch's formula: speed in MPH = C × √(HP ÷ weight), where C is a hull constant. Convert to knots by multiplying MPH by 0.869.
It reflects hull efficiency: roughly 150 for heavy displacement cruisers, 190 for typical runabouts, and 220+ for light high-performance race boats.
One knot equals about 1.151 MPH, so to convert MPH to knots you multiply by 0.869.
Crouch's formula is for planing hulls. Pure displacement hulls follow hull-speed limits based on waterline length instead.
Like cars, boats accelerate and plane based on power-to-weight. A heavier boat needs much more horsepower to reach the same speed.